ORCID Profile
0000-0003-3942-1702
Current Organisations
Dubai Health Authority
,
Deutsches Elektronen-Synchrotron DESY
Does something not look right? The information on this page has been harvested from data sources that may not be up to date. We continue to work with information providers to improve coverage and quality. To report an issue, use the Feedback Form.
Publisher: Springer Science and Business Media LLC
Date: 20-06-2023
DOI: 10.1140/EPJC/S10052-023-11559-Y
Abstract: This paper reports a search for Higgs boson pair ( hh ) production in association with a vector boson ( $$W\\ {\\text {o}r}\\ Z$$ W o r Z ) using 139 fb $$^{-1}$$ - 1 of proton–proton collision data at $$\\sqrt{s}=13\\,\\text {TeV}$$ s = 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. The search is performed in final states in which the vector boson decays leptonically ( $$W\\rightarrow \\ell \\nu ,\\, Z\\rightarrow \\ell \\ell ,\\nu \\nu $$ W → ℓ ν , Z → ℓ ℓ , ν ν with $$\\ell =e, \\mu $$ ℓ = e , μ ) and the Higgs bosons each decay into a pair of b -quarks. It targets Vhh signals from both non-resonant hh production, present in the Standard Model (SM), and resonant hh production, as predicted in some SM extensions. A 95% confidence-level upper limit of 183 (87) times the SM cross-section is observed (expected) for non-resonant Vhh production when assuming the kinematics are as expected in the SM. Constraints are also placed on Higgs boson coupling modifiers. For the resonant search, upper limits on the production cross-sections are derived for two specific models: one is the production of a vector boson along with a neutral heavy scalar resonance H , in the mass range 260–1000 GeV, that decays into hh , and the other is the production of a heavier neutral pseudoscalar resonance A that decays into a Z boson and H boson, where the A boson mass is 360–800 GeV and the H boson mass is 260–400 GeV. Constraints are also derived in the parameter space of two-Higgs-doublet models.
Publisher: Springer Science and Business Media LLC
Date: 28-07-2021
DOI: 10.1140/EPJC/S10052-023-11837-9
Abstract: A determination of the jet energy scale is presented using proton–proton collision data with a centre-of-mass energy of $$\\sqrt{s}=13$$ s = 13 TeV, corresponding to an integrated luminosity of 140 fb $$^{-1}$$ - 1 collected using the ATLAS detector at the LHC. Jets are reconstructed using the ATLAS particle-flow method that combines charged-particle tracks and topo-clusters formed from energy deposits in the calorimeter cells. The anti- $$k_\\textrm{t}$$ k t jet algorithm with radius parameter $$R=0.4$$ R = 0.4 is used to define the jet. Novel jet energy scale calibration strategies developed for the LHC Run 2 are reported that lay the foundation for the jet calibration in Run 3. Jets are calibrated with a series of simulation-based corrections, including state-of-the-art techniques in jet calibration such as machine learning methods and novel in situ calibrations to achieve better performance than the baseline calibration derived using up to 81 fb $$^{-1}$$ - 1 of Run 2 data. The performance of these new techniques is then examined in the in situ measurements by exploiting the transverse momentum balance between a jet and a reference object. The b -quark jet energy scale using particle flow jets is measured for the first time with around 1% precision using $$\\gamma $$ γ +jet events.
Publisher: Springer Science and Business Media LLC
Date: 05-2017
Publisher: Springer Science and Business Media LLC
Date: 07-2021
DOI: 10.1038/S41567-021-01225-Z
Abstract: Leptons with essentially the same properties apart from their mass are grouped into three families (or flavours). The number of leptons of each flavour is conserved in interactions, but this is not imposed by fundamental principles. Since the formulation of the standard model of particle physics, the observation of flavour oscillations among neutrinos has shown that lepton flavour is not conserved in neutrino weak interactions. So far, there has been no experimental evidence that this also occurs in interactions between charged leptons. Such an observation would be a sign of undiscovered particles or a yet unknown type of interaction. Here the ATLAS experiment at the Large Hadron Collider at CERN reports a constraint on lepton-flavour-violating effects in weak interactions, searching for Z -boson decays into a τ lepton and another lepton of different flavour with opposite electric charge. The branching fractions for these decays are measured to be less than 8.1 × 10 −6 ( e τ ) and 9.5 × 10 −6 ( μ τ ) at the 95% confidence level using 139 fb −1 of proton–proton collision data at a centre-of-mass energy of $$\\sqrt{s}=13\\,{\\rm{TeV}}$$ s = 13 TeV and 20.3 fb −1 at $$\\sqrt{s}=8\\,{\\rm{TeV}}.$$ s = 8 TeV . These results supersede the limits from the Large Electron–Positron Collider experiments conducted more than two decades ago.
Publisher: American Physical Society (APS)
Date: 15-05-2023
Publisher: Springer Science and Business Media LLC
Date: 05-2022
DOI: 10.1140/EPJC/S10052-022-10217-Z
Abstract: This paper presents an analysis at next-to-next-to-leading order in the theory of quantum chromodynamics for the determination of a new set of proton parton distribution functions using erse measurements in pp collisions at $$\\sqrt{s} = 7$$ s = 7 , 8 and 13 TeV, performed by the ATLAS experiment at the Large Hadron Collider, together with deep inelastic scattering data from ep collisions at the HERA collider. The ATLAS data sets considered are differential cross-section measurements of inclusive $$W^{\\pm }$$ W ± and $$Z/\\gamma ^*$$ Z / γ ∗ boson production, $$W^{\\pm }$$ W ± and Z boson production in association with jets, $$t\\bar{t}$$ t t ¯ production, inclusive jet production and direct photon production. In the analysis, particular attention is paid to the correlation of systematic uncertainties within and between the various ATLAS data sets and to the impact of model, theoretical and parameterisation uncertainties. The resulting set of parton distribution functions is called ATLASpdf21.
Publisher: Springer Science and Business Media LLC
Date: 31-10-2023
Publisher: Springer Science and Business Media LLC
Date: 2022
Abstract: A search for the exotic decay of the Higgs boson ( H ) into a b $$ \\overline{b} $$ b ¯ resonance plus missing transverse momentum is described. The search is performed with the ATLAS detector at the Large Hadron Collider using 139 fb − 1 of pp collisions at $$ \\sqrt{s} $$ s = 13 TeV. The search targets events from ZH production in an NMSSM scenario where H → $$ {\\overset{\\sim }{\\chi}}_2^0{\\overset{\\sim }{\\chi}}_1^0 $$ χ ~ 2 0 χ ~ 1 0 , with $$ {\\overset{\\sim }{\\chi}}_2^0 $$ χ ~ 2 0 → $$ a{\\overset{\\sim }{\\chi}}_1^0 $$ a χ ~ 1 0 , where a is a light pseudoscalar Higgs boson and $$ {\\overset{\\sim }{\\chi}}_{1,2}^0 $$ χ ~ 1 , 2 0 are the two lightest neutralinos. The decay of the a boson into a pair of b -quarks results in a peak in the dijet invariant mass distribution. The final-state signature consists of two leptons, two or more jets, at least one of which is identified as originating from a b -quark, and missing transverse momentum. Observations are consistent with Standard Model expectations and upper limits are set on the product of cross section times branching ratio for a three-dimensional scan of the masses of the $$ {\\overset{\\sim }{\\chi}}_2^0 $$ χ ~ 2 0 , $$ {\\overset{\\sim }{\\chi}}_1^0 $$ χ ~ 1 0 and a boson.
Publisher: Springer Science and Business Media LLC
Date: 04-07-2023
DOI: 10.1140/EPJC/S10052-023-11543-6
Abstract: A search for supersymmetry involving the pair production of gluinos decaying via off-shell third-generation squarks into the lightest neutralino $$(\\tilde{\\chi }^0_1)$$ ( χ ~ 1 0 ) is reported. It exploits LHC proton–proton collision data at a centre-of-mass energy $$\\sqrt{s} = 13$$ s = 13 TeV with an integrated luminosity of 139 fb $$^{-1}$$ - 1 collected with the ATLAS detector from 2015 to 2018. The search uses events containing large missing transverse momentum, up to one electron or muon, and several energetic jets, at least three of which must be identified as containing b -hadrons. Both a simple kinematic event selection and an event selection based upon a deep neural-network are used. No significant excess above the predicted background is found. In simplified models involving the pair production of gluinos that decay via off-shell top (bottom) squarks, gluino masses less than 2.44 TeV (2.35 TeV) are excluded at 95% CL for a massless $$\\tilde{\\chi }^0_1.$$ χ ~ 1 0 . Limits are also set on the gluino mass in models with variable branching ratios for gluino decays to $$b\\bar{b}\\tilde{\\chi }^0_1,$$ b b ¯ χ ~ 1 0 , $$t\\bar{t}\\tilde{\\chi }^0_1$$ t t ¯ χ ~ 1 0 and $$t\\bar{b}\\tilde{\\chi }^-_1/\\bar{t}b\\tilde{\\chi }^+_1.$$ t b ¯ χ ~ 1 - / t ¯ b χ ~ 1 + .
Publisher: Springer Science and Business Media LLC
Date: 06-2021
Abstract: Fiducial and differential cross-section measurements of W + W − production in association with at least one hadronic jet are presented. These measurements are sensitive to the properties of electroweak-boson self-interactions and provide a test of perturbative quantum chromodynamics and the electroweak theory. The analysis is performed using proton-proton collision data collected at $$ \sqrt{s} $$ s = 13 TeV with the ATLAS experiment, corresponding to an integrated luminosity of 139 fb − 1 . Events are selected with exactly one oppositely charged electron-muon pair and at least one hadronic jet with a transverse momentum of p T 30 GeV and a pseudorapidity of | η | 4 . 5. After subtracting the background contributions and correcting for detector effects, the jet-inclusive W + W − + ≥ 1 jet fiducial cross-section and W + W − + jets differential cross-sections with respect to several kinematic variables are measured. These measurements include leptonic quantities, such as the lepton transverse momenta and the transverse mass of the W + W − system, as well as jet-related observables such as the leading jet transverse momentum and the jet multiplicity. Limits on anomalous triple-gauge-boson couplings are obtained in a phase space where interference between the Standard Model litude and the anomalous litude is enhanced.
Publisher: American Physical Society (APS)
Date: 05-05-2023
Publisher: BMJ
Date: 24-05-2013
Publisher: American Physical Society (APS)
Date: 11-05-2023
Publisher: American Physical Society (APS)
Date: 05-09-2023
Publisher: Springer Science and Business Media LLC
Date: 2022
DOI: 10.1140/EPJC/S10052-021-09843-W
Abstract: A technique is presented to measure the efficiency with which c -jets are mistagged as b -jets (mistagging efficiency) using $$t\\bar{t}$$ t t ¯ events, where one of the W bosons decays into an electron or muon and a neutrino and the other decays into a quark–antiquark pair. The measurement utilises the relatively large and known $$W\\rightarrow cs$$ W → c s branching ratio, which allows a measurement to be made in an inclusive c -jet s le. The data s le used was collected by the ATLAS detector at $$\\sqrt{s} = 13$$ s = 13 $$\\text {TeV}$$ TeV and corresponds to an integrated luminosity of 139 fb $$^{-1}$$ - 1 . Events are reconstructed using a kinematic likelihood technique which selects the mapping between jets and $$t\\bar{t}$$ t t ¯ decay products that yields the highest likelihood value. The distribution of the b -tagging discriminant for jets from the hadronic W decays in data is compared with that in simulation to extract the mistagging efficiency as a function of jet transverse momentum. The total uncertainties are in the range 3–17%. The measurements generally agree with those in simulation but there are some differences in the region corresponding to the most stringent b -jet tagging requirement.
Publisher: Springer Science and Business Media LLC
Date: 19-06-2023
DOI: 10.1140/EPJC/S10052-023-11434-W
Abstract: Searches for new phenomena inspired by supersymmetry in final states containing an $$e^+e^-$$ e + e - or $$\\mu ^+\\mu ^-$$ μ + μ - pair, jets, and missing transverse momentum are presented. These searches make use of proton–proton collision data with an integrated luminosity of $$139~\\text {fb}^{-1}$$ 139 fb - 1 , collected during 2015–2018 at a centre-of-mass energy $$\\sqrt{s}=13~$$ s = 13 TeV by the ATLAS detector at the Large Hadron Collider. Two searches target the pair production of charginos and neutralinos. One uses the recursive-jigsaw reconstruction technique to follow up on excesses observed in $$36.1~\\text {fb}^{-1}$$ 36.1 fb - 1 of data, and the other uses conventional event variables. The third search targets pair production of coloured supersymmetric particles (squarks or gluinos) decaying through the next-to-lightest neutralino $$(\\tilde{\\chi }_2^0)$$ ( χ ~ 2 0 ) via a slepton $$(\\tilde{\\ell })$$ ( ℓ ~ ) or Z boson into $$\\ell ^+\\ell ^-\\tilde{\\chi }_1^0$$ ℓ + ℓ - χ ~ 1 0 , resulting in a kinematic endpoint or peak in the dilepton invariant mass spectrum. The data are found to be consistent with the Standard Model expectations. Results are interpreted using simplified models and exclude masses up to 900 GeV for electroweakinos, 1550 GeV for squarks, and 2250 GeV for gluinos.
Publisher: Springer Science and Business Media LLC
Date: 14-08-2023
DOI: 10.1140/EPJC/S10052-023-11736-Z
Abstract: The identification of b -jets, referred to as b -tagging, is an important part of many physics analyses in the ATLAS experiment at the Large Hadron Collider and an accurate calibration of its performance is essential for high-quality physics results. This publication describes the calibration of the light-flavour jet mistagging efficiency in a data s le of proton–proton collision events at $$\\sqrt{s}=13$$ s = 13 TeV corresponding to an integrated luminosity of 139 fb $$^{-1}$$ - 1 . The calibration is performed in a s le of Z bosons produced in association with jets. Due to the low mistagging efficiency for light-flavour jets, a method which uses modified versions of the b -tagging algorithms referred to as flip taggers is used in this work. A fit to the jet-flavour-sensitive secondary-vertex mass is performed to extract a scale factor from data, to correct the light-flavour jet mistagging efficiency in Monte Carlo simulations, while simultaneously correcting the b -jet efficiency. With this procedure, uncertainties coming from the modeling of jets from heavy-flavour hadrons are considerably lower than in previous calibrations of the mistagging scale factors, where they were dominant. The scale factors obtained in this calibration are consistent with unity within uncertainties.
Publisher: American Physical Society (APS)
Date: 18-09-2023
Publisher: Elsevier BV
Date: 2014
Publisher: Springer Science and Business Media LLC
Date: 10-2021
Abstract: A search for dark-matter particles in events with large missing transverse momentum and a Higgs boson candidate decaying into two photons is reported. The search uses 139 fb − 1 of proton-proton collision data collected at $$ \\sqrt{s} $$ s = 13 TeV with the ATLAS detector at the CERN LHC between 2015 and 2018. No significant excess of events over the Standard Model predictions is observed. The results are interpreted by extracting limits on three simplified models that include either vector or pseudoscalar mediators and predict a final state with a pair of dark-matter candidates and a Higgs boson decaying into two photons.
Publisher: Springer Science and Business Media LLC
Date: 19-04-2021
Abstract: A search for new phenomena with top quark pairs in final states with one isolated electron or muon, multiple jets, and large missing transverse momentum is performed. Signal regions are designed to search for two-, three-, and four-body decays of the directly pair-produced supersymmetric partner of the top quark (stop). Additional signal regions are designed specifically to search for spin-0 mediators that are produced in association with a pair of top quarks and decay into a pair of dark-matter particles. The search is performed using the Large Hadron Collider proton-proton collision dataset at a centre-of-mass energy of $$ \\sqrt{s} $$ s = 13 TeV recorded by the ATLAS detector from 2015 to 2018, corresponding to an integrated luminosity of 139 fb − 1 . No significant excess above the Standard Model background is observed, and limits at 95% confidence level are set in the stop-neutralino mass plane and as a function of the mediator mass or the dark-matter particle mass. Stops are excluded up to 1200 GeV (710 GeV) in the two-body (three-body) decay scenario. In the four-body scenario stops up to 640 GeV are excluded for a stop-neutralino mass difference of 60 GeV. Scalar and pseudoscalar dark-matter mediators are excluded up to 200 GeV when the coupling strengths of the mediator to Standard Model and dark-matter particles are both equal to one and when the mass of the dark-matter particle is 1 GeV.
Publisher: Springer Science and Business Media LLC
Date: 11-07-2022
DOI: 10.1140/EPJC/S10052-022-10489-5
Abstract: A search for long-lived charginos produced either directly or in the cascade decay of heavy prompt gluino states is presented. The search is based on proton–proton collision data collected at a centre-of-mass energy of $$\\sqrt{s}$$ s = 13 T $$\\text {eV}$$ eV between 2015 and 2018 with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 136 fb $$^{-1}$$ - 1 . Long-lived charginos are characterised by a distinct signature of a short and then disappearing track, and are reconstructed using at least four measurements in the ATLAS pixel detector, with no subsequent measurements in the silicon-microstrip tracking volume nor any associated energy deposits in the calorimeter. The final state is complemented by a large missing transverse-momentum requirement for triggering purposes and at least one high-transverse-momentum jet. No excess above the expected backgrounds is observed. Exclusion limits are set at 95% confidence level on the masses of the chargino and gluino for different chargino lifetimes. Chargino masses up to 660 (210) G $$\\text {eV}$$ eV are excluded in scenarios where the chargino is a pure wino (higgsino). For charginos produced during the cascade decay of a heavy gluino, gluinos with masses below 2.1 T $$\\text {eV}$$ eV are excluded for a chargino mass of 300 G $$\\text {eV}$$ eV and a lifetime of 0.2 ns.
Publisher: Springer Science and Business Media LLC
Date: 02-2021
Abstract: A search for the supersymmetric partners of quarks and gluons (squarks and gluinos) in final states containing jets and missing transverse momentum, but no electrons or muons, is presented. The data used in this search were recorded by the ATLAS experiment in proton-proton collisions at a centre-of-mass energy of $$ \\sqrt{s} $$ s = 13 TeV during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb − 1 . The results are interpreted in the context of various R -parity-conserving models where squarks and gluinos are produced in pairs or in association and a neutralino is the lightest supersymmetric particle. An exclusion limit at the 95% confidence level on the mass of the gluino is set at 2.30 TeV for a simplified model containing only a gluino and the lightest neutralino, assuming the latter is massless. For a simplified model involving the strong production of mass-degenerate first- and second-generation squarks, squark masses below 1.85 TeV are excluded if the lightest neutralino is massless. These limits extend substantially beyond the region of supersymmetric parameter space excluded previously by similar searches with the ATLAS detector.
Publisher: Springer Science and Business Media LLC
Date: 12-07-2023
DOI: 10.1140/EPJC/S10052-023-11578-9
Abstract: A search for pair production of doubly charged Higgs bosons ( $$H^{\\pm \\pm }$$ H ± ± ), each decaying into a pair of prompt, isolated, and highly energetic leptons with the same electric charge, is presented. The search uses a proton–proton collision data s le at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 139 fb $$^{-1}$$ - 1 recorded by the ATLAS detector during Run 2 of the Large Hadron Collider (LHC). This analysis focuses on same-charge leptonic decays, $$H^{\\pm \\pm } \\!\\rightarrow \\ell ^{\\pm } \\ell ^{\\prime \\pm }$$ H ± ± → ℓ ± ℓ ′ ± where $$\\ell , \\ell ^\\prime \\!=\\!e, \\mu , \\tau $$ ℓ , ℓ ′ = e , μ , τ , in two-, three-, and four-lepton channels, but only considers final states which include electrons or muons. No evidence of a signal is observed. Corresponding upper limits on the production cross-section of a doubly charged Higgs boson are derived, as a function of its mass $$m(H^{\\pm \\pm })$$ m ( H ± ± ) , at 95% confidence level. Assuming that the branching ratios to each of the possible leptonic final states are equal, $$\\mathcal {B}(H^{\\pm \\pm } \\rightarrow e^\\pm e^\\pm ) = \\mathcal {B}(H^{\\pm \\pm } \\rightarrow e^\\pm \\mu ^\\pm ) = \\mathcal {B}(H^{\\pm \\pm } \\rightarrow \\mu ^\\pm \\mu ^\\pm ) = \\mathcal {B}(H^{\\pm \\pm } \\rightarrow e^\\pm \\tau ^\\pm ) = \\mathcal {B}(H^{\\pm \\pm } \\rightarrow \\mu ^\\pm \\tau ^\\pm ) = \\mathcal {B}(H^{\\pm \\pm } \\rightarrow \\tau ^\\pm \\tau ^\\pm ) = 1/6$$ B ( H ± ± → e ± e ± ) = B ( H ± ± → e ± μ ± ) = B ( H ± ± → μ ± μ ± ) = B ( H ± ± → e ± τ ± ) = B ( H ± ± → μ ± τ ± ) = B ( H ± ± → τ ± τ ± ) = 1 / 6 , the observed (expected) lower limit on the mass of a doubly charged Higgs boson is 1080 GeV (1065 GeV) within the left-right symmetric type-II seesaw model, which is the strongest limit to date produced by the ATLAS Collaboration. Additionally, this paper provides the first direct test of the Zee–Babu neutrino mass model at the LHC, yielding an observed (expected) lower limit of $$m(H^{\\pm \\pm })$$ m ( H ± ± ) = 900 GeV (880 GeV).
Publisher: Springer Science and Business Media LLC
Date: 12-07-2023
DOI: 10.1140/EPJC/S10052-023-11582-Z
Abstract: This paper presents a search for dark matter, $$\\chi $$ χ , using events with a single top quark and an energetic W boson. The analysis is based on proton–proton collision data collected with the ATLAS experiment at $$\\sqrt{s}=$$ s = 13 TeV during LHC Run 2 (2015–2018), corresponding to an integrated luminosity of 139 fb $$^{-1}$$ - 1 . The search considers final states with zero or one charged lepton (electron or muon), at least one b -jet and large missing transverse momentum. In addition, a result from a previous search considering two-charged-lepton final states is included in the interpretation of the results. The data are found to be in good agreement with the Standard Model predictions and the results are interpreted in terms of 95% confidence-level exclusion limits in the context of a class of dark matter models involving an extended two-Higgs-doublet sector together with a pseudoscalar mediator particle. The search is particularly sensitive to on-shell production of the charged Higgs boson state, $$H^{\\pm }$$ H ± , arising from the two-Higgs-doublet mixing, and its semi-invisible decays via the mediator particle, a : $$H^{\\pm } \\rightarrow W^\\pm a (\\rightarrow \\chi \\chi )$$ H ± → W ± a ( → χ χ ) . Signal models with $$H^{\\pm }$$ H ± masses up to 1.5 TeV and a masses up to 350 GeV are excluded assuming a $$\\tan \\beta $$ tan β value of 1. For masses of a of 150 (250) GeV, $$\\tan \\beta $$ tan β values up to 2 are excluded for $$H^{\\pm }$$ H ± masses between 200 (400) GeV and 1.5 TeV. Signals with $$\\tan \\beta $$ tan β values between 20 and 30 are excluded for $$H^{\\pm }$$ H ± masses between 500 and 800 GeV.
Publisher: Springer Science and Business Media LLC
Date: 11-2020
DOI: 10.1140/EPJC/S10052-020-08509-3
Abstract: A search is presented for four-top-quark production using an integrated luminosity of 139 fb $$^{-1}$$ - 1 of proton–proton collision data at a centre-of-mass energy of $$13~\\text {TeV}$$ 13 TeV collected by the ATLAS detector at the LHC. Events are selected if they contain a same-sign lepton pair or at least three leptons (electrons or muons). Jet multiplicity, jet flavour and event kinematics are used to separate signal from the background through a multivariate discriminant, and dedicated control regions are used to constrain the dominant backgrounds. The four-top-quark production cross section is measured to be $$24^{+7}_{-6}$$ 24 - 6 + 7 fb. This corresponds to an observed (expected) significance with respect to the background-only hypothesis of 4.3 (2.4) standard deviations and provides evidence for this process.
Publisher: IOP Publishing
Date: 07-2021
DOI: 10.1088/1748-0221/16/07/P07029
Abstract: The ATLAS experiment at the Large Hadron Collider (LHC) employs a trigger system consisting of a first-level hardware trigger (L1) and a software-based high-level trigger. The L1 muon trigger system selects muon candidates, assigns them to the correct LHC bunch crossing and classifies them into one of six transverse-momentum threshold classes. The L1 muon trigger system uses resistive-plate chambers (RPCs) to generate the muon-induced trigger signals in the central (barrel) region of the ATLAS detector. The ATLAS RPCs are arranged in six concentric layers and operate in a toroidal magnetic field with a bending power of 1.5 to 5.5 Tm. The RPC detector consists of about 3700 gas volumes with a total surface area of more than 4000 m 2 . This paper reports on the performance of the RPC detector and L1 muon barrel trigger using 60.8 fb -1 of proton-proton collision data recorded by the ATLAS experiment in 2018 at a centre-of-mass energy of 13 TeV. Detector and trigger performance are studied using Z boson decays into a muon pair. Measurements of the RPC detector response, efficiency, and time resolution are reported. Measurements of the L1 muon barrel trigger efficiencies and rates are presented, along with measurements of the properties of the selected s le of muon candidates. Measurements of the RPC currents, counting rates and mean avalanche charge are performed using zero-bias collisions. Finally, RPC detector response and efficiency are studied at different high voltage and front-end discriminator threshold settings in order to extrapolate detector response to the higher luminosity expected for the High Luminosity LHC.
Publisher: Springer Science and Business Media LLC
Date: 11-2021
Abstract: A measurement of prompt photon-pair production in proton-proton collisions at $$ \\sqrt{s} $$ s = 13 TeV is presented. The data were recorded by the ATLAS detector at the LHC with an integrated luminosity of 139 fb − 1 . Events with two photons in the well-instrumented region of the detector are selected. The photons are required to be isolated and have a transverse momentum of $$ {p}_{\\mathrm{T}{,}_{\\gamma 1(2)}} $$ p T , γ 1 2 40 (30) GeV for the leading (sub-leading) photon. The differential cross sections as functions of several observables for the diphoton system are measured and compared with theoretical predictions from state-of-the-art Monte Carlo and fixed-order calculations. The QCD predictions from next-to-next-to-leading-order calculations and multi-leg merged calculations are able to describe the measured integrated and differential cross sections within uncertainties, whereas lower-order calculations show significant deviations, demonstrating that higher-order perturbative QCD corrections are crucial for this process. The resummed predictions with parton showers additionally provide an excellent description of the low transverse-momentum regime of the diphoton system.
Publisher: Elsevier BV
Date: 08-2023
Publisher: Springer Science and Business Media LLC
Date: 11-2021
Abstract: The production of dark matter in association with Higgs bosons is predicted in several extensions of the Standard Model. An exploration of such scenarios is presented, considering final states with missing transverse momentum and b -tagged jets consistent with a Higgs boson. The analysis uses proton-proton collision data at a centre-of-mass energy of 13 TeV recorded by the ATLAS experiment at the LHC during Run 2, amounting to an integrated luminosity of 139 fb − 1 . The analysis, when compared with previous searches, benefits from a larger dataset, but also has further improvements providing sensitivity to a wider spectrum of signal scenarios. These improvements include both an optimised event selection and advances in the object identification, such as the use of the likelihood-based significance of the missing transverse momentum and variable-radius track-jets. No significant deviation from Standard Model expectations is observed. Limits are set, at 95% confidence level, in two benchmark models with two Higgs doublets extended by either a heavy vector boson Z ′ or a pseudoscalar singlet a and which both provide a dark matter candidate χ . In the case of the two-Higgs-doublet model with an additional vector boson Z ′, the observed limits extend up to a Z ′ mass of 3 TeV for a mass of 100 GeV for the dark matter candidate. The two-Higgs-doublet model with a dark matter particle mass of 10 GeV and an additional pseudoscalar a is excluded for masses of the a up to 520 GeV and 240 GeV for tan β = 1 and tan β = 10 respectively. Limits on the visible cross-sections are set and range from to 0.05 fb to 3.26 fb, depending on the missing transverse momentum and b -quark jet multiplicity requirements.
Publisher: American Physical Society (APS)
Date: 08-08-2023
Publisher: American Physical Society (APS)
Date: 22-05-2023
Publisher: American Physical Society (APS)
Date: 31-01-2018
Publisher: Elsevier BV
Date: 11-2023
Publisher: Elsevier BV
Date: 03-2012
Publisher: Springer Science and Business Media LLC
Date: 14-08-2017
DOI: 10.1038/NPHYS4208
Publisher: Springer Science and Business Media LLC
Date: 04-2021
Abstract: The results of a search for direct pair production of top squarks and for dark matter in events with two opposite-charge leptons (electrons or muons), jets and missing transverse momentum are reported, using 139 fb − 1 of integrated luminosity from proton-proton collisions at $$ \\sqrt{s} $$ s = 13 TeV, collected by the ATLAS detector at the Large Hadron Collider during Run 2 (2015–2018). This search considers the pair production of top squarks and is sensitive across a wide range of mass differences between the top squark and the lightest neutralino. Additionally, spin-0 mediator dark-matter models are considered, in which the mediator is produced in association with a pair of top quarks. The mediator subsequently decays to a pair of dark-matter particles. No significant excess of events is observed above the Standard Model background, and limits are set at 95% confidence level. The results exclude top squark masses up to about 1 TeV, and masses of the lightest neutralino up to about 500 GeV. Limits on dark-matter production are set for scalar (pseudoscalar) mediator masses up to about 250 (300) GeV.
Publisher: Springer Science and Business Media LLC
Date: 06-2021
DOI: 10.1140/EPJC/S10052-021-09192-8
Abstract: The paper presents a measurement of the Standard Model Higgs Boson decaying to b -quark pairs in the vector boson fusion (VBF) production mode. A s le corresponding to 126 $$\\hbox {fb}^{-1}$$ fb - 1 of $$\\sqrt{s} = 13\\,\\text {TeV}$$ s = 13 TeV proton–proton collision data, collected with the ATLAS experiment at the Large Hadron Collider, is analyzed utilizing an adversarial neural network for event classification. The signal strength, defined as the ratio of the measured signal yield to that predicted by the Standard Model for VBF Higgs production, is measured to be $$0.95^{+0.38}_{-0.36}$$ 0 . 95 - 0.36 + 0.38 , corresponding to an observed (expected) significance of 2.6 (2.8) standard deviations from the background only hypothesis. The results are additionally combined with an analysis of Higgs bosons decaying to b -quarks, produced via VBF in association with a photon.
Publisher: Springer Science and Business Media LLC
Date: 05-2021
Abstract: The results of a search for new phenomena in final states with b -jets and missing transverse momentum using 139 fb − 1 of proton-proton data collected at a centre-of-mass energy $$ \\sqrt{s} $$ s = 13 TeV by the ATLAS detector at the LHC are reported. The analysis targets final states produced by the decay of a pair-produced supersymmetric bottom squark into a bottom quark and a stable neutralino. The analysis also seeks evidence for models of pair production of dark matter particles produced through the decay of a generic scalar or pseudoscalar mediator state in association with a pair of bottom quarks, and models of pair production of scalar third-generation down-type leptoquarks. No significant excess of events over the Standard Model background expectation is observed in any of the signal regions considered by the analysis. Bottom squark masses below 1270 GeV are excluded at 95% confidence level if the neutralino is massless. In the case of nearly mass-degenerate bottom squarks and neutralinos, the use of dedicated secondary-vertex identification techniques permits the exclusion of bottom squarks with masses up to 660 GeV for mass splittings between the squark and the neutralino of 10 GeV. These limits extend substantially beyond the regions of parameter space excluded by similar ATLAS searches performed previously.
Publisher: Springer Science and Business Media LLC
Date: 11-2021
Publisher: Elsevier BV
Date: 11-2017
Publisher: Springer Science and Business Media LLC
Date: 06-2021
Abstract: A search for charged Higgs bosons decaying into W ± W ± or W ± Z bosons is performed, involving experimental signatures with two leptons of the same charge, or three or four leptons with a variety of charge combinations, missing transverse momentum and jets. A data s le of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider between 2015 and 2018 is used. The data correspond to a total integrated luminosity of 139 fb − 1 . The search is guided by a type-II seesaw model that extends the scalar sector of the Standard Model with a scalar triplet, leading to a phenomenology that includes doubly and singly charged Higgs bosons. Two scenarios are explored, corresponding to the pair production of doubly charged H ±± bosons, or the associated production of a doubly charged H ±± boson and a singly charged H ± boson. No significant deviations from the Standard Model predictions are observed. H ±± bosons are excluded at 95% confidence level up to 350 GeV and 230 GeV for the pair and associated production modes, respectively.
Publisher: Springer Science and Business Media LLC
Date: 17-07-2023
DOI: 10.1140/EPJC/S10052-023-11700-X
Abstract: The exclusive production of pion pairs in the process $$pp\\rightarrow pp\\pi ^+\\pi ^-$$ p p → p p π + π - has been measured at $$\\sqrt{s}={7}\\,\\text {TeV}$$ s = 7 TeV with the ATLAS detector at the LHC, using $${80}\\,{\\upmu \\textrm{b}}^{-1}$$ 80 μ b - 1 of low-luminosity data. The pion pairs were detected in the ATLAS central detector while outgoing protons were measured in the forward ATLAS ALFA detector system. This represents the first use of proton tagging to measure an exclusive hadronic final state at the LHC. A cross-section measurement is performed in two kinematic regions defined by the proton momenta, the pion rapidities and transverse momenta, and the pion–pion invariant mass. Cross-section values of $$4.8 \\pm 1.0 \\mathrm {\\ (stat)} {~}^{+0.3}_{-0.2} \\mathrm {\\ (syst)}\\ {\\upmu \\textrm{b}}$$ 4.8 ± 1.0 ( stat ) - 0.2 + 0.3 ( syst ) μ b and $$9 \\pm 6 \\mathrm {\\ (stat)} {~}^{+2}_{-2} \\mathrm {\\ (syst)}\\ {\\upmu \\textrm{b}} $$ 9 ± 6 ( stat ) - 2 + 2 ( syst ) μ b are obtained in the two regions they are compared with theoretical models and provide a demonstration of the feasibility of measurements of this type.
Publisher: Springer Science and Business Media LLC
Date: 06-2021
Abstract: A search for charged Higgs bosons decaying into a top quark and a bottom quark is presented. The data analysed correspond to 139 fb − 1 of proton-proton collisions at $$ \\sqrt{s} $$ s = 13 TeV, recorded with the ATLAS detector at the LHC. The production of a heavy charged Higgs boson in association with a top quark and a bottom quark, pp → tbH + → tbtb , is explored in the H + mass range from 200 to 2000 GeV using final states with jets and one electron or muon. Events are categorised according to the multiplicity of jets and b -tagged jets, and multivariate analysis techniques are used to discriminate between signal and background events. No significant excess above the background-only hypothesis is observed and exclusion limits are derived for the production cross-section times branching ratio of a charged Higgs boson as a function of its mass they range from 3.6 pb at 200 GeV to 0.036 pb at 2000 GeV at 95% confidence level. The results are interpreted in the hMSSM and $$ {M}_h^{125} $$ M h 125 scenarios.
Publisher: Elsevier BV
Date: 08-2023
Publisher: American Physical Society (APS)
Date: 30-10-2023
Publisher: Springer Science and Business Media LLC
Date: 25-05-2023
DOI: 10.1140/EPJC/S10052-023-11427-9
Abstract: This paper presents a measurement of b -jet production in Pb+Pb and pp collisions at $$\\sqrt{s_{_\\text {NN}}}=5.02$$ s NN = 5.02 TeV with the ATLAS detector at the LHC. The measurement uses 260 $$\\text {pb}^{-1}$$ pb - 1 of pp collisions collected in 2017 and 1.4 $$\\text {nb}^{-1}$$ nb - 1 of Pb+Pb collisions collected in 2018. In both collision systems, jets are reconstructed via the anti- $$k_{t}$$ k t algorithm. The b -jets are identified from a s le of jets containing muons from the semileptonic decay of b -quarks using template fits of the muon momentum relative to the jet axis. In pp collisions, b -jets are reconstructed for radius parameters $$R= 0.2$$ R = 0.2 and $$R= 0.4$$ R = 0.4 , and only $$R= 0.2$$ R = 0.2 jets are used in Pb+Pb collisions. For comparison, inclusive $$R= 0.2$$ R = 0.2 jets are also measured using 1.7 $$\\text {nb}^{-1}$$ nb - 1 of Pb+Pb collisions collected in 2018 and the same pp collision data as the b -jet measurement. The nuclear modification factor, $$R_\\text {AA}$$ R AA , is calculated for both b -jets and inclusive jets with $$R= 0.2$$ R = 0.2 over the transverse momentum range of 80–290 GeV. The nuclear modification factor for b -jets decreases from peripheral to central collisions. The ratio of the b -jet $$R_\\text {AA}$$ R AA to inclusive jet $$R_\\text {AA}$$ R AA is also presented and suggests that the $$R_\\text {AA}$$ R AA for b -jets is larger than that for inclusive jets in central Pb+Pb collisions. The measurements are compared with theoretical calculations and suggest a role for mass and colour-charge effects in partonic energy loss in heavy-ion collisions.
Publisher: Springer Science and Business Media LLC
Date: 13-06-2023
DOI: 10.1140/EPJC/S10052-023-11477-Z
Abstract: This paper presents a statistical combination of searches targeting final states with two top quarks and invisible particles, characterised by the presence of zero, one or two leptons, at least one jet originating from a b -quark and missing transverse momentum. The analyses are searches for phenomena beyond the Standard Model consistent with the direct production of dark matter in pp collisions at the LHC, using 139 fb $$^{-\\text {1}}$$ - 1 of data collected with the ATLAS detector at a centre-of-mass energy of 13 TeV. The results are interpreted in terms of simplified dark matter models with a spin-0 scalar or pseudoscalar mediator particle. In addition, the results are interpreted in terms of upper limits on the Higgs boson invisible branching ratio, where the Higgs boson is produced according to the Standard Model in association with a pair of top quarks. For scalar (pseudoscalar) dark matter models, with all couplings set to unity, the statistical combination extends the mass range excluded by the best of the in idual channels by 50 (25) GeV, excluding mediator masses up to 370 GeV. In addition, the statistical combination improves the expected coupling exclusion reach by 14% (24%), assuming a scalar (pseudoscalar) mediator mass of 10 GeV. An upper limit on the Higgs boson invisible branching ratio of 0.38 ( $$\\text {0.30}^{+\\text {0.13}}_{-\\text {0.09}}$$ 0.30 - 0.09 + 0.13 ) is observed (expected) at 95% confidence level.
Publisher: American Physical Society (APS)
Date: 16-08-2023
Publisher: Springer Science and Business Media LLC
Date: 11-2020
Abstract: In this paper, a new technique for reconstructing and identifying hadronically decaying τ + τ − pairs with a large Lorentz boost, referred to as the di- τ tagger, is developed and used for the first time in the ATLAS experiment at the Large Hadron Collider. A benchmark di- τ tagging selection is employed in the search for resonant Higgs boson pair production, where one Higgs boson decays into a boosted $$ b\\overline{b} $$ b b ¯ pair and the other into a boosted τ + τ − pair, with two hadronically decaying τ -leptons in the final state. Using 139 fb − 1 of proton-proton collision data recorded at a centre-of-mass energy of 13 TeV, the efficiency of the di- τ tagger is determined and the background with quark- or gluon-initiated jets misidentified as di- τ objects is estimated. The search for a heavy, narrow, scalar resonance produced via gluon-gluon fusion and decaying into two Higgs bosons is carried out in the mass range 1–3 TeV using the same dataset. No deviations from the Standard Model predictions are observed, and 95% confidence-level exclusion limits are set on this model.
Publisher: Elsevier BV
Date: 09-2012
Publisher: Elsevier BV
Date: 07-2023
Publisher: Springer Science and Business Media LLC
Date: 03-2021
DOI: 10.1140/EPJC/S10052-021-08970-8
Abstract: A Correction to this paper has been published: 10.1140/epjc/s10052-020-08730-0
Publisher: Springer Science and Business Media LLC
Date: 04-07-2022
DOI: 10.1038/S41586-022-04893-W
Abstract: The standard model of particle physics 1–4 describes the known fundamental particles and forces that make up our Universe, with the exception of gravity. One of the central features of the standard model is a field that permeates all of space and interacts with fundamental particles 5–9 . The quantum excitation of this field, known as the Higgs field, manifests itself as the Higgs boson, the only fundamental particle with no spin. In 2012, a particle with properties consistent with the Higgs boson of the standard model was observed by the ATLAS and CMS experiments at the Large Hadron Collider at CERN 10,11 . Since then, more than 30 times as many Higgs bosons have been recorded by the ATLAS experiment, enabling much more precise measurements and new tests of the theory. Here, on the basis of this larger dataset, we combine an unprecedented number of production and decay processes of the Higgs boson to scrutinize its interactions with elementary particles. Interactions with gluons, photons, and W and Z bosons—the carriers of the strong, electromagnetic and weak forces—are studied in detail. Interactions with three third-generation matter particles (bottom ( b ) and top ( t ) quarks, and tau leptons ( τ )) are well measured and indications of interactions with a second-generation particle (muons, μ ) are emerging. These tests reveal that the Higgs boson discovered ten years ago is remarkably consistent with the predictions of the theory and provide stringent constraints on many models of new phenomena beyond the standard model.
Publisher: Springer Science and Business Media LLC
Date: 27-01-2022
DOI: 10.1007/S41781-021-00062-2
Abstract: The accurate simulation of additional interactions at the ATLAS experiment for the analysis of proton–proton collisions delivered by the Large Hadron Collider presents a significant challenge to the computing resources. During the LHC Run 2 (2015–2018), there were up to 70 inelastic interactions per bunch crossing, which need to be accounted for in Monte Carlo (MC) production. In this document, a new method to account for these additional interactions in the simulation chain is described. Instead of s ling the inelastic interactions and adding their energy deposits to a hard-scatter interaction one-by-one, the inelastic interactions are pres led, independent of the hard scatter, and stored as combined events. Consequently, for each hard-scatter interaction, only one such pres led event needs to be added as part of the simulation chain. For the Run 2 simulation chain, with an average of 35 interactions per bunch crossing, this new method provides a substantial reduction in MC production CPU needs of around 20%, while reproducing the properties of the reconstructed quantities relevant for physics analyses with good accuracy.
Publisher: American Physical Society (APS)
Date: 16-10-2023
Publisher: Elsevier BV
Date: 10-2016
Publisher: Springer Science and Business Media LLC
Date: 27-06-2023
DOI: 10.1140/EPJC/S10052-023-11579-8
Abstract: Cross-sections for the production of a Z boson in association with two photons are measured in proton–proton collisions at a centre-of-mass energy of 13 TeV. The data used correspond to an integrated luminosity of 139 fb $$^{-1}$$ - 1 recorded by the ATLAS experiment during Run 2 of the LHC. The measurements use the electron and muon decay channels of the Z boson, and a fiducial phase-space region where the photons are not radiated from the leptons. The integrated $$Z(\\rightarrow \\ell \\ell )\\gamma \\gamma $$ Z ( → ℓ ℓ ) γ γ cross-section is measured with a precision of 12% and differential cross-sections are measured as a function of six kinematic variables of the $$Z\\gamma \\gamma $$ Z γ γ system. The data are compared with predictions from MC event generators which are accurate to up to next-to-leading order in QCD. The cross-section measurements are used to set limits on the coupling strengths of dimension-8 operators in the framework of an effective field theory.
Publisher: Springer Science and Business Media LLC
Date: 04-07-2023
DOI: 10.1140/EPJC/S10052-023-11583-Y
Abstract: A study of the charge conjugation and parity ( $$\\textit{CP}$$ CP ) properties of the interaction between the Higgs boson and $$\\tau $$ τ -leptons is presented. The study is based on a measurement of $$\\textit{CP}$$ CP -sensitive angular observables defined by the visible decay products of $$\\tau $$ τ -leptons produced in Higgs boson decays. The analysis uses 139 fb $$^{-1}$$ - 1 of proton–proton collision data recorded at a centre-of-mass energy of $$\\sqrt{s}= 13$$ s = 13 TeV with the ATLAS detector at the Large Hadron Collider. Contributions from $$\\textit{CP}$$ CP -violating interactions between the Higgs boson and $$\\tau $$ τ -leptons are described by a single mixing angle parameter $$\\phi _{\\tau }$$ ϕ τ in the generalised Yukawa interaction. Without constraining the $$H\\rightarrow \\tau \\tau $$ H → τ τ signal strength to its expected value under the Standard Model hypothesis, the mixing angle $$\\phi _{\\tau }$$ ϕ τ is measured to be $$9^{\\circ } \\pm 16^{\\circ }$$ 9 ∘ ± 16 ∘ , with an expected value of $$0^{\\circ } \\pm 28^{\\circ }$$ 0 ∘ ± 28 ∘ at the 68% confidence level. The pure $$\\textit{CP}$$ CP -odd hypothesis is disfavoured at a level of 3.4 standard deviations. The results are compatible with the predictions for the Higgs boson in the Standard Model.
Publisher: Springer Science and Business Media LLC
Date: 10-2020
Abstract: This paper presents a search for new heavy particles decaying into a pair of top quarks using 139 fb − 1 of proton-proton collision data recorded at a centre-of-mass energy of $$ \\sqrt{s} $$ s = 13 TeV with the ATLAS detector at the Large Hadron Collider. The search is performed using events consistent with pair production of high-transverse-momentum top quarks and their subsequent decays into the fully hadronic final states. The analysis is optimized for resonances decaying into a $$ t\\overline{t} $$ t t ¯ pair with mass above 1.4 TeV, exploiting a dedicated multivariate technique with jet substructure to identify hadronically decaying top quarks using large-radius jets and evaluating the background expectation from data. No significant deviation from the background prediction is observed. Limits are set on the production cross-section times branching fraction for the new Z ′ boson in a topcolor-assisted-technicolor model. The Z ′ boson masses below 3.9 and 4.7 TeV are excluded at 95% confidence level for the decay widths of 1% and 3%, respectively.
Publisher: Springer Science and Business Media LLC
Date: 03-2021
DOI: 10.1140/EPJC/S10052-021-08929-9
Abstract: A search for the pair production of heavy leptons as predicted by the type-III seesaw mechanism is presented. The search uses proton–proton collision data at a centre-of-mass energy of 13 TeV, corresponding to $$ 139\\,{\\text {fb}}^{-1} $$ 139 fb - 1 of integrated luminosity recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis focuses on the final state with two light leptons (electrons or muons) of different flavour and charge combinations, with at least two jets and large missing transverse momentum. No significant excess over the Standard Model expectation is observed. The results are translated into exclusion limits on heavy-lepton masses, and the observed lower limit on the mass of the type-III seesaw heavy leptons is 790 GeV at 95% confidence level.
Publisher: IOP Publishing
Date: 07-2021
DOI: 10.1088/1748-0221/16/07/P07006
Abstract: The ATLAS Fast TracKer (FTK) was designed to provide full tracking for the ATLAS high-level trigger by using pattern recognition based on Associative Memory (AM) chips and fitting in high-speed field programmable gate arrays. The tracks found by the FTK are based on inputs from all modules of the pixel and silicon microstrip trackers. The as-built FTK system and components are described, as is the online software used to control them while running in the ATLAS data acquisition system. Also described is the simulation of the FTK hardware and the optimization of the AM pattern banks. An optimization for long-lived particles with large impact parameter values is included. A test of the FTK system with the data playback facility that allowed the FTK to be commissioned during the shutdown between Run 2 and Run 3 of the LHC is reported. The resulting tracks from part of the FTK system covering a limited η–ϕ region of the detector are compared with the output from the FTK simulation. It is shown that FTK performance is in good agreement with the simulation.
Publisher: Springer Science and Business Media LLC
Date: 03-11-2022
DOI: 10.1140/EPJC/S10052-022-10785-0
Abstract: A search for the pair production of heavy leptons as predicted by the type-III seesaw mechanism is presented. The search uses proton–proton collision data at a centre-of-mass energy of 13 TeV, corresponding to $$139\\,\\text {fb}^{-1}$$ 139 fb - 1 of integrated luminosity recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis focuses on final states with three or four electrons or muons from the possible decays of new heavy leptons via intermediate electroweak bosons. No significant deviations above the Standard Model expectation are observed upper and lower limits on the heavy lepton production cross-section and masses are derived respectively. These results are then combined for the first time with the ones already published by ATLAS using the channel with two leptons in the final state. The observed lower limit on the mass of the type-III seesaw heavy leptons combining two, three and four lepton channels together is 910 GeV at the 95% confidence level.
Publisher: Springer Science and Business Media LLC
Date: 11-2021
Abstract: A novel search for exotic decays of the Higgs boson into pairs of long-lived neutral particles, each decaying into a bottom quark pair, is performed using 139 fb − 1 of $$ \\sqrt{s} $$ s = 13 TeV proton-proton collision data collected with the ATLAS detector at the LHC. Events consistent with the production of a Higgs boson in association with a leptonically decaying Z boson are analysed. Long-lived particle (LLP) decays are reconstructed from inner-detector tracks as displaced vertices with high mass and track multiplicity relative to Standard Model processes. The analysis selection requires the presence of at least two displaced vertices, effectively suppressing Standard Model backgrounds. The residual background contribution is estimated using a data-driven technique. No excess over Standard Model predictions is observed, and upper limits are set on the branching ratio of the Higgs boson to LLPs. Branching ratios above 10% are excluded at 95% confidence level for LLP mean proper lifetimes cτ as small as 4 mm and as large as 100 mm. For LLP masses below 40 GeV, these results represent the most stringent constraint in this lifetime regime.
Publisher: Springer Science and Business Media LLC
Date: 08-2020
DOI: 10.1140/EPJC/S10052-020-8181-6
Abstract: A measurement of observables sensitive to spin correlations in $$t\\bar{t}$$ t t ¯ production is presented, using 36.1 $$\\hbox {fb}^{-1}$$ fb - 1 of pp collision data at $$\\sqrt{s} = 13$$ s = 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. Differential cross-sections are measured in events with exactly one electron and one muon with opposite-sign electric charge as a function of the azimuthal opening angle and the absolute difference in pseudorapidity between the electron and muon candidates in the laboratory frame. The azimuthal opening angle is also measured as a function of the invariant mass of the $$t\\bar{t}$$ t t ¯ system. The measured differential cross-sections are compared to predictions by several NLO Monte Carlo generators and fixed-order calculations. The observed degree of spin correlation is somewhat higher than predicted by the generators used. The data are consistent with the prediction of one of the fixed-order calculations at NLO, but agree less well with higher-order predictions. Using these leptonic observables, a search is performed for pair production of supersymmetric top squarks decaying into Standard Model top quarks and light neutralinos. Top squark masses between 170 and 230 GeV are largely excluded at the 95% confidence level for kinematically allowed values of the neutralino mass.
Publisher: Elsevier BV
Date: 08-2023
Publisher: IOP Publishing
Date: 2022
DOI: 10.1088/1748-0221/17/01/P01013
Abstract: The semiconductor tracker (SCT) is one of the tracking systems for charged particles in the ATLAS detector. It consists of 4088 silicon strip sensor modules. During Run 2 (2015–2018) the Large Hadron Collider delivered an integrated luminosity of 156 fb -1 to the ATLAS experiment at a centre-of-mass proton-proton collision energy of 13 TeV. The instantaneous luminosity and pile-up conditions were far in excess of those assumed in the original design of the SCT detector. Due to improvements to the data acquisition system, the SCT operated stably throughout Run 2. It was available for 99.9% of the integrated luminosity and achieved a data-quality efficiency of 99.85%. Detailed studies have been made of the leakage current in SCT modules and the evolution of the full depletion voltage, which are used to study the impact of radiation damage to the modules.
Publisher: Springer Science and Business Media LLC
Date: 07-2021
Abstract: Measurements of four-lepton differential and integrated fiducial cross-sections in events with two same-flavour, opposite-charge electron or muon pairs are presented. The data correspond to 139 fb − 1 of $$ \\sqrt{s} $$ s = 13 TeV proton-proton collisions, collected by the ATLAS detector during Run 2 of the Large Hadron Collider (2015–2018). The final state has contributions from a number of interesting Standard Model processes that dominate in different four-lepton invariant mass regions, including single Z boson production, Higgs boson production and on-shell ZZ production, with a complex mix of interference terms, and possible contributions from physics beyond the Standard Model. The differential cross-sections include the four-lepton invariant mass inclusively, in slices of other kinematic variables, and in different lepton flavour categories. Also measured are dilepton invariant masses, transverse momenta, and angular correlation variables, in four regions of four-lepton invariant mass, each dominated by different processes. The measurements are corrected for detector effects and are compared with state-of-the-art Standard Model calculations, which are found to be consistent with the data. The Z → 4 ℓ branching fraction is extracted, giving a value of (4 . 41 ± 0 . 30) × 10 − 6 . Constraints on effective field theory parameters and a model based on a spontaneously broken B − L gauge symmetry are also evaluated. Further reinterpretations can be performed with the provided information.
Publisher: Elsevier BV
Date: 07-2023
Publisher: Springer Science and Business Media LLC
Date: 12-2020
DOI: 10.1140/EPJC/S10052-020-08554-Y
Abstract: This paper reports on a search for heavy resonances decaying into WW , ZZ or WZ using proton–proton collision data at a centre-of-mass energy of $$\\sqrt{s}=13$$ s = 13 TeV. The data, corresponding to an integrated luminosity of 139 $$\\mathrm{fb}^{1}$$ fb 1 , were recorded with the ATLAS detector from 2015 to 2018 at the Large Hadron Collider. The search is performed for final states in which one W or Z boson decays leptonically, and the other W boson or Z boson decays hadronically. The data are found to be described well by expected backgrounds. Upper bounds on the production cross sections of heavy scalar, vector or tensor resonances are derived in the mass range 300–5000 GeV within the context of Standard Model extensions with warped extra dimensions or including a heavy vector triplet. Production through gluon–gluon fusion, Drell–Yan or vector-boson fusion are considered, depending on the assumed model.
Publisher: MDPI AG
Date: 09-09-2021
Abstract: When graduates of Australian social work courses embark on a career in mental health, the systems they enter are complex, fragmented and evolving. Emerging practitioners will commonly be confronted by the loneliness, social exclusion, poverty and prejudice experienced by people living with mental distress however, social work practice may not be focused on these factors. Instead, in accordance with the dominant biomedical perspective, symptom and risk management may predominate. Frustration with the limitations evident in this approach has seen the United Nations call for the transformation of mental health service delivery. Recognising paradigmatic influences on mental health social work may lead to a more considered enactment of person centred, recovery and rights-based approaches. This paper compares and contrasts influences of neo-liberalism, critical theory, human rights and post-structuralism on mental health social work practice. In preparing social work practitioners to recognise the influence of, and work more creatively with, intersecting paradigms, social work educators strive to foster a transformative approach to mental health practice that straddles discourses.
Publisher: Springer Science and Business Media LLC
Date: 04-2021
Abstract: The Lagrangian in eq. (2.1) of JHEP 11 (2020) 005 has an erroneous factor of 1/2. The analysis code used for the results reported in the paper does not have this factor. The results remain unchanged.
Publisher: American Physical Society (APS)
Date: 07-08-2023
Publisher: IOP Publishing
Date: 04-04-2013
Publisher: BMJ
Date: 03-04-2014
Publisher: Springer Science and Business Media LLC
Date: 10-08-2023
DOI: 10.1140/EPJC/S10052-023-11790-7
Abstract: A search for pair-produced vector-like quarks using events with exactly one lepton ( e or $$\\mu $$ μ ), at least four jets including at least one b -tagged jet, and large missing transverse momentum is presented. Data from proton–proton collisions at a centre-of-mass energy of $$\\sqrt{s}=$$ s = 13 $$\\text {TeV}$$ TeV , recorded by the ATLAS detector at the LHC from 2015 to 2018 and corresponding to an integrated luminosity of 139 fb $$^{-1}$$ - 1 , are analysed. Vector-like partners T and B of the top and bottom quarks are considered, as is a vector-like X with charge $$+5/3$$ + 5 / 3 , assuming their decay into a W , Z , or Higgs boson and a third-generation quark. No significant deviations from the Standard Model expectation are observed. Upper limits on the production cross-section of T and B quark pairs as a function of their mass are derived for various decay branching ratio scenarios. The strongest lower limits on the masses are 1.59 $$\\text {TeV}$$ TeV assuming mass-degenerate vector-like quarks and branching ratios corresponding to the weak-isospin doublet model, and 1.47 $$\\text {TeV}$$ TeV (1.46 $$\\text {TeV}$$ TeV ) for exclusive $$T \\rightarrow Zt$$ T → Z t ( $$B/X \\rightarrow Wt$$ B / X → W t ) decays. In addition, lower limits on the T and B quark masses are derived for all possible branching ratios.
Publisher: Springer Science and Business Media LLC
Date: 03-2021
Abstract: This paper describes a measurement of light-by-light scattering based on Pb+Pb collision data recorded by the ATLAS experiment during Run 2 of the LHC. The study uses 2 . 2 nb − 1 of integrated luminosity collected in 2015 and 2018 at $$ \sqrt{s_{\mathrm{NN}}} $$ s NN = 5 . 02 TeV. Light-by-light scattering candidates are selected in events with two photons produced exclusively, each with transverse energy $$ {E}_{\mathrm{T}}^{\gamma } $$ E T γ 2 . 5 GeV, pseudorapidity |η γ | 2 . 37, diphoton invariant mass m γγ 5 GeV, and with small diphoton transverse momentum and diphoton acoplanarity. The integrated and differential fiducial cross sections are measured and compared with theoretical predictions. The diphoton invariant mass distribution is used to set limits on the production of axion-like particles. This result provides the most stringent limits to date on axion-like particle production for masses in the range 6–100 GeV. Cross sections above 2 to 70 nb are excluded at the 95% CL in that mass interval.
Publisher: Springer Science and Business Media LLC
Date: 08-2020
DOI: 10.1140/EPJC/S10052-020-8050-3
Abstract: The results of a search for electroweakino pair production $$pp \\rightarrow \\tilde{\\chi }^\\pm _1 \\tilde{\\chi }^0_2$$ p p → χ ~ 1 ± χ ~ 2 0 in which the chargino ( $$\\tilde{\\chi }^\\pm _1$$ χ ~ 1 ± ) decays into a W boson and the lightest neutralino ( $$\\tilde{\\chi }^0_1$$ χ ~ 1 0 ), while the heavier neutralino ( $$\\tilde{\\chi }^0_2$$ χ ~ 2 0 ) decays into the Standard Model 125 GeV Higgs boson and a second $$\\tilde{\\chi }^0_1$$ χ ~ 1 0 are presented. The signal selection requires a pair of b -tagged jets consistent with those from a Higgs boson decay, and either an electron or a muon from the W boson decay, together with missing transverse momentum from the corresponding neutrino and the stable neutralinos. The analysis is based on data corresponding to 139 $$\\mathrm {fb}^{-1}$$ fb - 1 of $$\\sqrt{s}=13$$ s = 13 TeV pp collisions provided by the Large Hadron Collider and recorded by the ATLAS detector. No statistically significant evidence of an excess of events above the Standard Model expectation is found. Limits are set on the direct production of the electroweakinos in simplified models, assuming pure wino cross-sections. Masses of $$\\tilde{\\chi }^{\\pm }_{1}/\\tilde{\\chi }^{0}_{2}$$ χ ~ 1 ± / χ ~ 2 0 up to 740 GeV are excluded at 95% confidence level for a massless $$\\tilde{\\chi }^{0}_{1}$$ χ ~ 1 0 .
Publisher: Springer Science and Business Media LLC
Date: 10-2020
DOI: 10.1140/EPJC/S10052-020-8223-0
Abstract: Inclusive and differential fiducial cross sections of the Higgs boson are measured in the $$H \\rightarrow ZZ^{*} \\rightarrow 4\\ell $$ H → Z Z ∗ → 4 ℓ ( $$\\ell = e,\\mu $$ ℓ = e , μ ) decay channel. The results are based on proton-proton collision data produced at the Large Hadron Collider at a centre-of-mass energy of 13 TeV and recorded by the ATLAS detector from 2015 to 2018, equivalent to an integrated luminosity of 139 $$\\hbox {fb}^{-1}$$ fb - 1 . The inclusive fiducial cross section for the $$H \\rightarrow ZZ^{*} \\rightarrow 4\\ell $$ H → Z Z ∗ → 4 ℓ process is measured to be $$\\sigma _\\mathrm {fid} = 3.28 \\,{\\pm }\\, 0.32$$ σ fid = 3.28 ± 0.32 fb, in agreement with the Standard Model prediction of $$\\sigma _\\mathrm {fid, SM} = 3.41 \\pm 0.18 $$ σ fid , SM = 3.41 ± 0.18 fb. Differential fiducial cross sections are measured for a variety of observables which are sensitive to the production and decay of the Higgs boson. All measurements are in agreement with the Standard Model predictions. The results are used to constrain anomalous Higgs boson interactions with Standard Model particles.
Publisher: Elsevier BV
Date: 08-2023
Publisher: IOP Publishing
Date: 08-2021
DOI: 10.1088/1748-0221/16/08/P08025
Abstract: Non-ionizing energy loss causes bulk damage to the silicon sensors of the ATLAS pixel and strip detectors. This damage has important implications for data-taking operations, charged-particle track reconstruction, detector simulations, and physics analysis. This paper presents simulations and measurements of the leakage current in the ATLAS pixel detector and semiconductor tracker as a function of location in the detector and time, using data collected in Run 1 (2010–2012) and Run 2 (2015–2018) of the Large Hadron Collider. The extracted fluence shows a much stronger | z |-dependence in the innermost layers than is seen in simulation. Furthermore, the overall fluence on the second innermost layer is significantly higher than in simulation, with better agreement in layers at higher radii. These measurements are important for validating the simulation models and can be used in part to justify safety factors for future detector designs and interventions.
Publisher: Springer Science and Business Media LLC
Date: 02-2018
Publisher: Springer Science and Business Media LLC
Date: 18-08-2022
DOI: 10.1140/EPJC/S10052-022-10588-3
Abstract: A search for the Higgs boson decaying into a pair of charm quarks is presented. The analysis uses proton–proton collisions to target the production of a Higgs boson in association with a leptonically decaying W or Z boson. The dataset delivered by the LHC at a centre-of-mass energy of "Equation missing" and recorded by the ATLAS detector corresponds to an integrated luminosity of 139 $$\text{ fb}^{-1}$$ fb - 1 . Flavour-tagging algorithms are used to identify jets originating from the hadronisation of charm quarks. The analysis method is validated with the simultaneous measurement of WW , WZ and ZZ production, with observed (expected) significances of 2.6 (2.2) standard deviations above the background-only prediction for the $$(W/Z)Z(\rightarrow c{\bar{c}})$$ ( W / Z ) Z ( → c c ¯ ) process and 3.8 (4.6) standard deviations for the $$(W/Z)W(\rightarrow cq)$$ ( W / Z ) W ( → c q ) process. The $$(W/Z)H(\rightarrow c {\bar{c}})$$ ( W / Z ) H ( → c c ¯ ) search yields an observed (expected) upper limit of 26 (31) times the predicted Standard Model cross-section times branching fraction for a Higgs boson with a mass of "Equation missing" , corresponding to an observed (expected) constraint on the charm Yukawa coupling modifier $$|\kappa _c| 8.5~(12.4)$$ | κ c | 8.5 ( 12.4 ) , at the 95% confidence level. A combination with the ATLAS $$(W/Z)H, H\rightarrow b{\bar{b}}$$ ( W / Z ) H , H → b b ¯ analysis is performed, allowing the ratio $$\kappa _c / \kappa _b$$ κ c / κ b to be constrained to less than 4.5 at the 95% confidence level, smaller than the ratio of the b- and c-quark masses, and therefore determines the Higgs-charm coupling to be weaker than the Higgs-bottom coupling at the 95% confidence level.
Publisher: Springer Science and Business Media LLC
Date: 11-2021
Abstract: A measurement of four-top-quark production using proton-proton collision data at a centre-of-mass energy of 13 TeV collected by the ATLAS detector at the Large Hadron Collider corresponding to an integrated luminosity of 139 fb − 1 is presented. Events are selected if they contain a single lepton (electron or muon) or an opposite-sign lepton pair, in association with multiple jets. The events are categorised according to the number of jets and how likely these are to contain b -hadrons. A multivariate technique is then used to discriminate between signal and background events. The measured four-top-quark production cross section is found to be $$ {26}_{-15}^{+17} $$ 26 − 15 + 17 fb, with a corresponding observed (expected) significance of 1.9 (1.0) standard deviations over the background-only hypothesis. The result is combined with the previous measurement performed by the ATLAS Collaboration in the multilepton final state. The combined four-top-quark production cross section is measured to be $$ {24}_{-6}^{+7} $$ 24 − 6 + 7 fb, with a corresponding observed (expected) signal significance of 4.7 (2.6) standard deviations over the background-only predictions. It is consistent within 2.0 standard deviations with the Standard Model expectation of 12 . 0 ± 2 . 4 fb.
Publisher: Springer Science and Business Media LLC
Date: 10-2020
DOI: 10.1140/EPJC/S10052-020-8227-9
Abstract: Higgs boson properties are studied in the four-lepton decay channel (where lepton = e , $$\\mu $$ μ ) using 139 $$\\hbox {fb}^{-1}$$ fb - 1 of proton–proton collision data recorded at $$\\sqrt{s}=$$ s = 13 TeV by the ATLAS experiment at the Large Hadron Collider. The inclusive cross-section times branching ratio for $$H\\rightarrow ZZ^*$$ H → Z Z ∗ decay is measured to be $$1.34 \\pm 0.12$$ 1.34 ± 0.12 pb for a Higgs boson with absolute rapidity below 2.5, in good agreement with the Standard Model prediction of $$1.33 \\pm 0.08$$ 1.33 ± 0.08 pb. Cross-sections times branching ratio are measured for the main Higgs boson production modes in several exclusive phase-space regions. The measurements are interpreted in terms of coupling modifiers and of the tensor structure of Higgs boson interactions using an effective field theory approach. Exclusion limits are set on the CP-even and CP-odd ‘beyond the Standard Model’ couplings of the Higgs boson to vector bosons, gluons and top quarks.
Publisher: Springer Science and Business Media LLC
Date: 03-2022
Abstract: Searches are conducted for new spin-0 or spin-1 bosons using events where a Higgs boson with mass 125 GeV decays into four leptons ( ℓ = e , μ ). This decay is presumed to occur via an intermediate state which contains two on-shell, promptly decaying bosons: H → XX/ZX → 4 ℓ , where the new boson X has a mass between 1 and 60 GeV. The search uses pp collision data collected with the ATLAS detector at the LHC with an integrated luminosity of 139 fb − 1 at a centre-of-mass energy $$ \\sqrt{s} $$ s = 13 TeV. The data are found to be consistent with Standard Model expectations. Limits are set on fiducial cross sections and on the branching ratio of the Higgs boson to decay into XX/ZX , improving those from previous publications by a factor between two and four. Limits are also set on mixing parameters relevant in extensions of the Standard Model containing a dark sector where X is interpreted to be a dark boson.
Publisher: Springer Science and Business Media LLC
Date: 05-09-2023
Publisher: IOP Publishing
Date: 19-05-2011
Publisher: Springer Science and Business Media LLC
Date: 08-2020
DOI: 10.1140/EPJC/S10052-020-8102-8
Abstract: A search for direct pair production of scalar partners of the top quark (top squarks or scalar third-generation up-type leptoquarks) in the all-hadronic $$t{\\bar{t}}$$ t t ¯ plus missing transverse momentum final state is presented. The analysis of 139 $$\\hbox {fb}^{-1}$$ fb - 1 of $${\\sqrt{s}=13}$$ s = 13 TeV proton–proton collision data collected using the ATLAS detector at the LHC yields no significant excess over the Standard Model background expectation. To interpret the results, a supersymmetric model is used where the top squark decays via $${\\tilde{t}} \\rightarrow t^{(*)} {\\tilde{\\chi }}^0_1$$ t ~ → t ( ∗ ) χ ~ 1 0 , with $$t^{(*)}$$ t ( ∗ ) denoting an on-shell (off-shell) top quark and $${\\tilde{\\chi }}^0_1$$ χ ~ 1 0 the lightest neutralino. Three specific event selections are optimised for the following scenarios. In the scenario where $$m_{{\\tilde{t}}} m_t+m_{{\\tilde{\\chi }}^0_1}$$ m t ~ m t + m χ ~ 1 0 , top squark masses are excluded in the range 400–1250 GeV for $${\\tilde{\\chi }}^0_1$$ χ ~ 1 0 masses below 200 GeV at 95% confidence level. In the situation where $$m_{{\\tilde{t}}}\\sim m_t+m_{{\\tilde{\\chi }}^0_1}$$ m t ~ ∼ m t + m χ ~ 1 0 , top squark masses in the range 300–630 GeV are excluded, while in the case where $$m_{{\\tilde{t}}} m_W+m_b+m_{{\\tilde{\\chi }}^0_1}$$ m t ~ m W + m b + m χ ~ 1 0 (with $$m_{{\\tilde{t}}}-m_{{\\tilde{\\chi }}^0_1}\\ge 5$$ m t ~ - m χ ~ 1 0 ≥ 5 GeV), considered for the first time in an ATLAS all-hadronic search, top squark masses in the range 300–660 GeV are excluded. Limits are also set for scalar third-generation up-type leptoquarks, excluding leptoquarks with masses below 1240 GeV when considering only leptoquark decays into a top quark and a neutrino.
Publisher: Springer Science and Business Media LLC
Date: 08-2023
DOI: 10.1140/EPJC/S10052-023-11584-X
Abstract: This paper presents the muon momentum calibration and performance studies for the ATLAS detector based on the pp collisions data s le produced at $$\\sqrt{s}$$ s = 13 TeV at the LHC during Run 2 and corresponding to an integrated luminosity of 139 $${\\textrm{fb}}^{-1}$$ fb - 1 . An innovative approach is used to correct for potential charge-dependent momentum biases related to the knowledge of the detector geometry, using the $$Z\\rightarrow \\mu ^{+}\\mu ^{-}$$ Z → μ + μ - resonance. The muon momentum scale and resolution are measured using s les of $$J/\\psi \\rightarrow \\mu ^{+}\\mu ^{-}$$ J / ψ → μ + μ - and $$Z\\rightarrow \\mu ^{+}\\mu ^{-}$$ Z → μ + μ - events. A calibration procedure is defined and applied to simulated data to match the performance measured in real data. The calibration is validated using an independent s le of $$\\Upsilon \\rightarrow \\mu ^{+}\\mu ^{-}$$ Υ → μ + μ - events. At the Z $$(J/\\psi )$$ ( J / ψ ) peak, the momentum scale is measured with an uncertainty at the 0.05% (0.1%) level, and the resolution is measured with an uncertainty at the 1.5% (2%) level. The charge-dependent bias is removed with a dedicated in situ correction for momenta up to 450 GeV with a precision better than 0.03 $${\\textrm{TeV}}^{-1}$$ TeV - 1 .
Publisher: Springer Science and Business Media LLC
Date: 02-2021
Abstract: A search for dark matter is conducted in final states containing a photon and missing transverse momentum in proton-proton collisions at $$ \\sqrt{s} $$ s = 13 TeV. The data, collected during 2015–2018 by the ATLAS experiment at the CERN LHC, correspond to an integrated luminosity of 139 fb − 1 . No deviations from the predictions of the Standard Model are observed and 95% confidence-level upper limits between 2.45 fb and 0.5 fb are set on the visible cross section for contributions from physics beyond the Standard Model, in different ranges of the missing transverse momentum. The results are interpreted as 95% confidence-level limits in models where weakly interacting dark-matter candidates are pair-produced via an s -channel axial-vector or vector mediator. Dark-matter candidates with masses up to 415 (580) GeV are excluded for axial-vector (vector) mediators, while the maximum excluded mass of the mediator is 1460 (1470) GeV. In addition, the results are expressed in terms of 95% confidence-level limits on the parameters of a model with an axion-like particle produced in association with a photon, and are used to constrain the coupling g a Z γ of an axion-like particle to the electroweak gauge bosons.
Publisher: American Physical Society (APS)
Date: 09-08-2023
Publisher: Springer Science and Business Media LLC
Date: 30-10-2021
Abstract: A search for pair production of third-generation scalar leptoquarks decaying into a top quark and a τ -lepton is presented. The search is based on a dataset of pp collisions at $$ \\sqrt{s} $$ s = 13 TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb − 1 . Events are selected if they have one light lepton (electron or muon) and at least one hadronically decaying τ -lepton, or at least two light leptons. In addition, two or more jets, at least one of which must be identified as containing b -hadrons, are required. Six final states, defined by the multiplicity and flavour of lepton candidates, are considered in the analysis. Each of them is split into multiple event categories to simultaneously search for the signal and constrain several leading backgrounds. The signal-rich event categories require at least one hadronically decaying τ -lepton candidate and exploit the presence of energetic final-state objects, which is characteristic of signal events. No significant excess above the Standard Model expectation is observed in any of the considered event categories, and 95% CL upper limits are set on the production cross section as a function of the leptoquark mass, for different assumptions about the branching fractions into tτ and bν . Scalar leptoquarks decaying exclusively into tτ are excluded up to masses of 1 . 43 TeV while, for a branching fraction of 50% into tτ , the lower mass limit is 1 . 22 TeV.
Publisher: Springer Science and Business Media LLC
Date: 12-2021
Abstract: The fragmentation properties of jets containing b -hadrons are studied using charged B mesons in 139 fb − 1 of pp collisions at $$ \\sqrt{s} $$ s = 13 TeV, recorded with the ATLAS detector at the LHC during the period from 2015 to 2018. The B mesons are reconstructed using the decay of B ± into J/ψK ± , with the J/ψ decaying into a pair of muons. Jets are reconstructed using the anti- k t algorithm with radius parameter R = 0 . 4. The measurement determines the longitudinal and transverse momentum profiles of the reconstructed B hadrons with respect to the axes of the jets to which they are geometrically associated. These distributions are measured in intervals of the jet transverse momentum, ranging from 50 GeV to above 100 GeV. The results are corrected for detector effects and compared with several Monte Carlo predictions using different parton shower and hadronisation models. The results for the longitudinal and transverse profiles provide useful inputs to improve the description of heavy-flavour fragmentation in jets.
Publisher: Springer Science and Business Media LLC
Date: 23-01-2118
Publisher: Springer Science and Business Media LLC
Date: 07-2021
Abstract: This article presents a new set of proton parton distribution functions, ATLASepWZVjet20, produced in an analysis at next-to-next-to-leading order in QCD. The new data sets considered are the measurements of W + and W − boson and Z boson production in association with jets in pp collisions at $$ \\sqrt{s} $$ s = 8 TeV performed by the ATLAS experiment at the LHC with integrated luminosities of 20 . 2 fb − 1 and 19 . 9 fb − 1 , respectively. The analysis also considers the ATLAS measurements of differential W ± and Z boson production at $$ \\sqrt{s} $$ s = 7 TeV with an integrated luminosity of 4 . 6 fb − 1 and deep-inelastic-scattering data from e ± p collisions at the HERA accelerator. An improved determination of the sea-quark densities at high Bjorken x is shown, while confirming a strange-quark density similar in size to the up- and down-sea-quark densities in the range x ≲ 0 . 02 found by previous ATLAS analyses.
Publisher: Springer Science and Business Media LLC
Date: 04-2022
DOI: 10.1140/EPJC/S10052-022-10101-W
Abstract: A measurement of the energy asymmetry in jet-associated top-quark pair production is presented using $$139\\,{\\mathrm {fb}}^{-1}$$ 139 fb - 1 of data collected by the ATLAS detector at the Large Hadron Collider during pp collisions at $$\\sqrt{s}=13\\,\\text {TeV}$$ s = 13 TeV . The observable measures the different probability of top and antitop quarks to have the higher energy as a function of the jet scattering angle with respect to the beam axis. The energy asymmetry is measured in the semileptonic $$t{\\bar{t}}$$ t t ¯ decay channel, and the hadronically decaying top quark must have transverse momentum above $$350\\,\\text {GeV}$$ 350 GeV . The results are corrected for detector effects to particle level in three bins of the scattering angle of the associated jet. The measurement agrees with the SM prediction at next-to-leading-order accuracy in quantum chromodynamics in all three bins. In the bin with the largest expected asymmetry, where the jet is emitted perpendicular to the beam, the energy asymmetry is measured to be $$-0.043\\pm 0.020$$ - 0.043 ± 0.020 , in agreement with the SM prediction of $$-0.037\\pm 0.003$$ - 0.037 ± 0.003 . Interpreting this result in the framework of the Standard Model effective field theory (SMEFT), it is shown that the energy asymmetry is sensitive to the top-quark chirality in four-quark operators and is therefore a valuable new observable in global SMEFT fits.
Publisher: Springer Science and Business Media LLC
Date: 09-2023
Publisher: Springer Science and Business Media LLC
Date: 2021
Abstract: A measurement of event-shape variables in proton-proton collisions at large momentum transfer is presented using data collected at $$ \\sqrt{s} $$ s = 13 TeV with the ATLAS detector at the Large Hadron Collider. Six event-shape variables calculated using hadronic jets are studied in inclusive multijet events using data corresponding to an integrated luminosity of 139 fb − 1 . Measurements are performed in bins of jet multiplicity and in different ranges of the scalar sum of the transverse momenta of the two leading jets, reaching scales beyond 2 TeV. These measurements are compared with predictions from Monte Carlo event generators containing leading-order or next-to-leading order matrix elements matched to parton showers simulated to leading-logarithm accuracy. At low jet multiplicities, shape discrepancies between the measurements and the Monte Carlo predictions are observed. At high jet multiplicities, the shapes are better described but discrepancies in the normalisation are observed.
Publisher: Springer Science and Business Media LLC
Date: 18-07-2022
DOI: 10.1140/EPJC/S10052-022-10366-1
Abstract: This article presents the results of two studies of Higgs boson properties using the $$WW^*(\\rightarrow e\\nu \\mu \\nu )jj$$ W W ∗ ( → e ν μ ν ) j j final state, based on a dataset corresponding to $${36.1}{{\\mathrm{fb}}^{-1}}$$ 36.1 fb - 1 of $$\\sqrt{s}=13$$ s = 13 TeV proton–proton collisions recorded by the ATLAS experiment at the Large Hadron Collider. The first study targets Higgs boson production via gluon–gluon fusion and constrains the CP properties of the effective Higgs–gluon interaction. Using angular distributions and the overall rate, a value of $$\\tan (\\alpha ) = 0.0 \\pm 0.4 (\\mathrm {stat.}) \\pm 0.3 (\\mathrm {syst.})$$ tan ( α ) = 0.0 ± 0.4 ( stat . ) ± 0.3 ( syst . ) is obtained for the tangent of the mixing angle for CP-even and CP-odd contributions. The second study exploits the vector-boson fusion production mechanism to probe the Higgs boson couplings to longitudinally and transversely polarised W and Z bosons in both the production and the decay of the Higgs boson these couplings have not been directly constrained previously. The polarisation-dependent coupling-strength scale factors are defined as the ratios of the measured polarisation-dependent coupling strengths to those predicted by the Standard Model, and are determined using rate and kinematic information to be $$a_\\mathrm {L}=0.91^{+0.10}_{-0.18}$$ a L = 0 . 91 - 0.18 + 0.10 (stat.) $$^{+0.09}_{-0.17}$$ - 0.17 + 0.09 (syst.) and $$a_{\\mathrm {T}}=1.2 \\pm 0.4 $$ a T = 1.2 ± 0.4 (stat.) $$ ^{+0.2}_{-0.3} $$ - 0.3 + 0.2 (syst.). These coupling strengths are translated into pseudo-observables, resulting in $$\\kappa _{VV}= 0.91^{+0.10}_{-0.18}$$ κ VV = 0 . 91 - 0.18 + 0.10 (stat.) $$^{+0.09}_{-0.17}$$ - 0.17 + 0.09 (syst.) and $$\\epsilon _{VV} =0.13^{+0.28}_{-0.20}$$ ϵ VV = 0 . 13 - 0.20 + 0.28 (stat.) $$^{+0.08}_{-0.10}$$ - 0.10 + 0.08 (syst.). All results are consistent with the Standard Model predictions.
Publisher: Springer Science and Business Media LLC
Date: 26-05-2023
DOI: 10.1140/EPJC/S10052-023-11436-8
Abstract: In a special run of the LHC with $$\\beta ^{\\star } = 2.5$$ β ⋆ = 2.5 km, proton–proton elastic-scattering events were recorded at $$\\sqrt{s} = 13$$ s = 13 TeV with an integrated luminosity of $$340~\\upmu {\\text {b}}^{-1}$$ 340 μ b - 1 using the ALFA subdetector of ATLAS in 2016. The elastic cross section was measured differentially in the Mandelstam t variable in the range from $$-t = 2.5 \\cdot 10^{-4}$$ - t = 2.5 · 10 - 4 GeV $$^{2}$$ 2 to $$-t = 0.46$$ - t = 0.46 GeV $$^{2}$$ 2 using 6.9 million elastic-scattering candidates. This paper presents measurements of the total cross section $$\\sigma _{\\text {tot}}$$ σ tot , parameters of the nuclear slope, and the $$\\rho $$ ρ -parameter defined as the ratio of the real part to the imaginary part of the elastic-scattering litude in the limit $$t \\rightarrow 0$$ t → 0 . These parameters are determined from a fit to the differential elastic cross section using the optical theorem and different parameterizations of the t -dependence. The results for $$\\sigma _{\\text {tot}}$$ σ tot and $$\\rho $$ ρ are $$\\begin{aligned} \\sigma _{\\text {tot}}(pp\\rightarrow X) = 104.7 \\pm 1.1 \\ \\text{ mb },\\quad \\rho = 0.098 \\pm 0.011 . \\end{aligned}$$ σ tot ( p p → X ) = 104.7 ± 1.1 mb , ρ = 0.098 ± 0.011 . The uncertainty in $$\\sigma _{\\text {tot}}$$ σ tot is dominated by the luminosity measurement, and in $$\\rho $$ ρ by imperfect knowledge of the detector alignment and by modelling of the nuclear litude.
Publisher: BMJ
Date: 25-03-2014
Publisher: Springer Science and Business Media LLC
Date: 10-08-2023
DOI: 10.1140/EPJC/S10052-023-11508-9
Abstract: This paper presents a measurement of fiducial and differential cross-sections for $$W^{+}W^{-}$$ W + W - production in proton–proton collisions at $$\\sqrt{s}=13$$ s = 13 TeV with the ATLAS experiment at the Large Hadron Collider using a dataset corresponding to an integrated luminosity of 139 fb $$^{-1}$$ - 1 . Events with exactly one electron, one muon and no hadronic jets are studied. The fiducial region in which the measurements are performed is inspired by searches for the electroweak production of supersymmetric charginos decaying to two-lepton final states. The selected events have moderate values of missing transverse momentum and the ‘stransverse mass’ variable $$m_{\\textrm{T2}}$$ m T2 , which is widely used in searches for supersymmetry at the LHC. The ranges of these variables are chosen so that the acceptance is enhanced for direct $$W^{+}W^{-}$$ W + W - production and suppressed for production via top quarks, which is treated as a background. The fiducial cross-section and particle-level differential cross-sections for six variables are measured and compared with two theoretical SM predictions from perturbative QCD calculations.
Publisher: American Physical Society (APS)
Date: 19-07-2023
Publisher: Elsevier BV
Date: 03-2011
Publisher: Springer Science and Business Media LLC
Date: 03-2021
Abstract: A search is presented for the production of the Standard Model Higgs boson in association with a high-energy photon. With a focus on the vector-boson fusion process and the dominant Higgs boson decay into b -quark pairs, the search benefits from a large reduction of multijet background compared to more inclusive searches. Results are reported from the analysis of 132 fb − 1 of pp collision data at $$ \\sqrt{s} $$ s = 13 TeV collected with the ATLAS detector at the LHC. The measured Higgs boson signal yield in this final-state signature is 1 . 3 ± 1 . 0 times the Standard Model prediction. The observed significance of the Higgs boson signal above the background is 1 . 3 standard deviations, compared to an expected significance of 1 . 0 standard deviations.
Publisher: Springer Science and Business Media LLC
Date: 02-2021
DOI: 10.1140/EPJC/S10052-020-08734-W
Abstract: Differential cross-section measurements are presented for the electroweak production of two jets in association with a Z boson. These measurements are sensitive to the vector-boson fusion production mechanism and provide a fundamental test of the gauge structure of the Standard Model. The analysis is performed using proton–proton collision data collected by ATLAS at $$\\sqrt{s}=13\\ \\hbox {TeV}$$ s = 13 TeV and with an integrated luminosity of $$139\\ \\hbox {fb}^{-1}$$ 139 fb - 1 . The differential cross-sections are measured in the $$Z\\rightarrow \\ell ^+\\ell ^-$$ Z → ℓ + ℓ - decay channel ( $$\\ell =e,\\mu $$ ℓ = e , μ ) as a function of four observables: the dijet invariant mass, the rapidity interval spanned by the two jets, the signed azimuthal angle between the two jets, and the transverse momentum of the dilepton pair. The data are corrected for the effects of detector inefficiency and resolution and are sufficiently precise to distinguish between different state-of-the-art theoretical predictions calculated using Powheg+Pythia8 , Herwig7+Vbfnlo and Sherpa 2.2. The differential cross-sections are used to search for anomalous weak-boson self-interactions using a dimension-six effective field theory. The measurement of the signed azimuthal angle between the two jets is found to be particularly sensitive to the interference between the Standard Model and dimension-six scattering litudes and provides a direct test of charge-conjugation and parity invariance in the weak-boson self-interactions.
Publisher: Springer Science and Business Media LLC
Date: 04-2022
DOI: 10.1140/EPJC/S10052-022-10182-7
Abstract: A search is presented for the production of a single top quark via left-handed flavour-changing neutral-current (FCNC) interactions of a top quark, a gluon and an up or charm quark. Two production processes are considered: $$u+g\rightarrow t$$ u + g → t and $$c+g\rightarrow t$$ c + g → t . The analysis is based on proton–proton collision data taken at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC. The data set corresponds to an integrated luminosity of 139 fb $$^{-1}$$ - 1 . Events with exactly one electron or muon, exactly one b -tagged jet and missing transverse momentum are selected, resembling the decay products of a singly produced top quark. Neural networks based on kinematic variables differentiate between events from the two signal processes and events from background processes. The measured data are consistent with the background-only hypothesis, and limits are set on the production cross-sections of the signal processes: $$\sigma (u+g\rightarrow t)\times \mathcal {B}(t\rightarrow Wb)\times \mathcal {B}(W\rightarrow \ell \nu ) .0\,$$ σ ( u + g → t ) × B ( t → W b ) × B ( W → ℓ ν ) 3.0 pb and $$\sigma (c+g\rightarrow t)\times \mathcal {B}(t\rightarrow Wb)\times \mathcal {B}(W\rightarrow \ell \nu ) .7\,$$ σ ( c + g → t ) × B ( t → W b ) × B ( W → ℓ ν ) 4.7 pb at the 95% confidence level, with $$\mathcal {B}(W\rightarrow \ell \nu )=0.325$$ B ( W → ℓ ν ) = 0.325 being the sum of branching ratios of all three leptonic decay modes of the W boson. Based on the framework of an effective field theory, the cross-section limits are translated into limits on the strengths of the tug and tcg couplings occurring in the theory: $$|C^{\,ut}_{uG}|/\Lambda ^2 0.057\,$$ | C uG u t | / Λ 2 0.057 TeV $$^{-2}$$ - 2 and $$|C^{\,ct}_{uG}|/\Lambda ^2 0.14\,$$ | C uG c t | / Λ 2 0.14 TeV $$^{-2}$$ - 2 . These bounds correspond to limits on the branching ratios of FCNC-induced top-quark decays: $$\mathcal {B}(t\rightarrow u+g) 0.61\times 10^{-4}$$ B ( t → u + g ) 0.61 × 10 - 4 and $$\mathcal {B}(t\rightarrow c+g) 3.7\times 10^{-4}$$ B ( t → c + g ) 3.7 × 10 - 4 .
Publisher: Springer Science and Business Media LLC
Date: 11-2017
Publisher: Springer Science and Business Media LLC
Date: 08-2021
DOI: 10.1140/EPJC/S10052-021-09402-3
Abstract: Jet energy scale and resolution measurements with their associated uncertainties are reported for jets using 36–81 fb $$^{-1}$$ - 1 of proton–proton collision data with a centre-of-mass energy of $$\sqrt{s}=13$$ s = 13 $${\text {Te}}{\text {V}}$$ TeV collected by the ATLAS detector at the LHC. Jets are reconstructed using two different input types: topo-clusters formed from energy deposits in calorimeter cells, as well as an algorithmic combination of charged-particle tracks with those topo-clusters, referred to as the ATLAS particle-flow reconstruction method. The anti- $$k_t$$ k t jet algorithm with radius parameter $$R=0.4$$ R = 0.4 is the primary jet definition used for both jet types. This result presents new jet energy scale and resolution measurements in the high pile-up conditions of late LHC Run 2 as well as a full calibration of particle-flow jets in ATLAS. Jets are initially calibrated using a sequence of simulation-based corrections. Next, several in situ techniques are employed to correct for differences between data and simulation and to measure the resolution of jets. The systematic uncertainties in the jet energy scale for central jets ( $$|\eta | .2$$ | η | 1.2 ) vary from 1% for a wide range of high- $$p_{{\text {T}}}$$ p T jets ( $$250 _{{\text {T}}} ~{\text {Ge}}{\text {V}}$$ 250 p T 2000 GeV ), to 5% at very low $$p_{{\text {T}}}$$ p T ( $$20~{\text {Ge}}{\text {V}}$$ 20 GeV ) and 3.5% at very high $$p_{{\text {T}}}$$ p T ( $$ .5~{\text {Te}}{\text {V}}$$ 2.5 TeV ). The relative jet energy resolution is measured and ranges from ( $$24 \pm 1.5$$ 24 ± 1.5 )% at 20 $${\text {Ge}}{\text {V}}$$ GeV to ( $$6 \pm 0.5$$ 6 ± 0.5 )% at 300 $${\text {Ge}}{\text {V}}$$ GeV .
Publisher: Elsevier BV
Date: 05-2015
Publisher: Springer Science and Business Media LLC
Date: 11-03-2022
DOI: 10.1007/S41781-021-00079-7
Abstract: The ATLAS experiment at the Large Hadron Collider has a broad physics programme ranging from precision measurements to direct searches for new particles and new interactions, requiring ever larger and ever more accurate datasets of simulated Monte Carlo events. Detector simulation with Geant4 is accurate but requires significant CPU resources. Over the past decade, ATLAS has developed and utilized tools that replace the most CPU-intensive component of the simulation—the calorimeter shower simulation—with faster simulation methods. Here, AtlFast3, the next generation of high-accuracy fast simulation in ATLAS, is introduced. AtlFast3 combines parameterized approaches with machine-learning techniques and is deployed to meet current and future computing challenges, and simulation needs of the ATLAS experiment. With highly accurate performance and significantly improved modelling of substructure within jets, AtlFast3 can simulate large numbers of events for a wide range of physics processes.
Publisher: American Physical Society (APS)
Date: 30-08-2023
Publisher: Springer Science and Business Media LLC
Date: 20-06-2023
DOI: 10.1140/EPJC/S10052-023-11479-X
Abstract: A measurement of observables sensitive to effects of colour reconnection in top-quark pair-production events is presented using 139 $$\\hbox {fb}^{-1}$$ fb - 1 of 13 TeV proton–proton collision data collected by the ATLAS detector at the LHC. Events are selected by requiring exactly one isolated electron and one isolated muon with opposite charge and two or three jets, where exactly two jets are required to be b -tagged. For the selected events, measurements are presented for the charged-particle multiplicity, the scalar sum of the transverse momenta of the charged particles, and the same scalar sum in bins of charged-particle multiplicity. These observables are unfolded to the stable-particle level, thereby correcting for migration effects due to finite detector resolution, acceptance and efficiency effects. The particle-level measurements are compared with different colour reconnection models in Monte Carlo generators. These measurements disfavour some of the colour reconnection models and provide inputs to future optimisation of the parameters in Monte Carlo generators.
Publisher: American Physical Society (APS)
Date: 25-08-2023
Publisher: American Physical Society (APS)
Date: 21-08-2023
Publisher: Elsevier BV
Date: 08-2023
Publisher: Elsevier BV
Date: 11-2023
Publisher: Springer Science and Business Media LLC
Date: 31-07-2023
DOI: 10.1140/EPJC/S10052-023-11699-1
Abstract: The flavour-tagging algorithms developed by the ATLAS Collaboration and used to analyse its dataset of $$\\sqrt{s} = 13$$ s = 13 TeV pp collisions from Run 2 of the Large Hadron Collider are presented. These new tagging algorithms are based on recurrent and deep neural networks, and their performance is evaluated in simulated collision events. These developments yield considerable improvements over previous jet-flavour identification strategies. At the 77% b -jet identification efficiency operating point, light-jet (charm-jet) rejection factors of 170 (5) are achieved in a s le of simulated Standard Model $$t\\bar{t}$$ t t ¯ events similarly, at a c -jet identification efficiency of 30%, a light-jet ( b -jet) rejection factor of 70 (9) is obtained.
Publisher: Elsevier BV
Date: 11-2023
Publisher: American Physical Society (APS)
Date: 14-08-2023
Publisher: American Physical Society (APS)
Date: 18-08-2023
Publisher: Springer Science and Business Media LLC
Date: 11-07-2022
DOI: 10.1140/EPJC/S10052-022-10472-0
Abstract: This paper presents studies of Bose–Einstein correlations (BEC) in proton–proton collisions at a centre-of-mass energy of 13 TeV, using data from the ATLAS detector at the CERN Large Hadron Collider. Data were collected in a special low-luminosity configuration with a minimum-bias trigger and a high-multiplicity track trigger, accumulating integrated luminosities of 151 $$\upmu $$ μ b $$^{-1}$$ - 1 and 8.4 nb $$^{-1}$$ - 1 , respectively. The BEC are measured for pairs of like-sign charged particles, each with $$|\eta | 2.5$$ | η | 2.5 , for two kinematic ranges: the first with particle $$p_{\mathrm {T}} 100$$ p T 100 MeV and the second with particle $$p_{\mathrm {T}} 500$$ p T 500 MeV. The BEC parameters, characterizing the source radius and particle correlation strength, are investigated as functions of charged-particle multiplicity (up to 300) and average transverse momentum of the pair (up to 1.5 GeV). The double-differential dependence on charged-particle multiplicity and average transverse momentum of the pair is also studied. The BEC radius is found to be independent of the charged-particle multiplicity for high charged-particle multiplicity (above 100), confirming a previous observation at lower energy. This saturation occurs independent of the transverse momentum of the pair.
Publisher: Springer Science and Business Media LLC
Date: 11-2020
Abstract: A search for new physics with non-resonant signals in dielectron and dimuon final states in the mass range above 2 TeV is presented. This is the first search for non-resonant signals in dilepton final states at the LHC to use a background estimate from the data. The data, corresponding to an integrated luminosity of 139 fb − 1 , were recorded by the ATLAS experiment in proton-proton collisions at a center-of-mass energy of $$ \\sqrt{s} $$ s = 13 TeV during Run 2 of the Large Hadron Collider. The benchmark signal signature is a two-quark and two-lepton contact interaction, which would enhance the dilepton event rate at the TeV mass scale. To model the contribution from background processes a functional form is fit to the dilepton invariant-mass spectra in data in a mass region below the region of interest. It is then extrapolated to a high-mass signal region to obtain the expected background there. No significant deviation from the expected background is observed in the data. Upper limits at 95% CL on the number of events and the visible cross-section times branching fraction for processes involving new physics are provided. Observed (expected) 95% CL lower limits on the contact interaction energy scale reach 35.8 (37.6) TeV.
Publisher: Elsevier BV
Date: 10-2023
Publisher: Springer Science and Business Media LLC
Date: 02-2021
DOI: 10.1140/EPJC/S10052-020-08677-2
Abstract: Measurements of the Standard Model Higgs boson decaying into a $$b\\bar{b}$$ b b ¯ pair and produced in association with a W or Z boson decaying into leptons, using proton–proton collision data collected between 2015 and 2018 by the ATLAS detector, are presented. The measurements use collisions produced by the Large Hadron Collider at a centre-of-mass energy of $$\\sqrt{s} = 13\\,\\text {Te}\\text {V}$$ s = 13 Te , corresponding to an integrated luminosity of $$139\\,\\mathrm {fb}^{-1}$$ 139 fb - 1 . The production of a Higgs boson in association with a W or Z boson is established with observed (expected) significances of 4.0 (4.1) and 5.3 (5.1) standard deviations, respectively. Cross-sections of associated production of a Higgs boson decaying into bottom quark pairs with an electroweak gauge boson, W or Z , decaying into leptons are measured as a function of the gauge boson transverse momentum in kinematic fiducial volumes. The cross-section measurements are all consistent with the Standard Model expectations, and the total uncertainties vary from 30% in the high gauge boson transverse momentum regions to 85% in the low regions. Limits are subsequently set on the parameters of an effective Lagrangian sensitive to modifications of the WH and ZH processes as well as the Higgs boson decay into $$b\\bar{b}$$ b b ¯ .
Publisher: Springer Science and Business Media LLC
Date: 07-2021
DOI: 10.1038/S41567-021-01236-W
Abstract: The standard model of particle physics encapsulates our best current understanding of physics at the smallest scales. A fundamental axiom of this theory is the universality of the couplings of the different generations of leptons to the electroweak gauge bosons. The measurement of the ratio of the decay rate of W bosons to τ leptons and muons, R ( τ / μ ), constitutes an important test of this axiom. Using 139 fb −1 of proton–proton collisions recorded with the ATLAS detector at a centre-of-mass energy of 13 TeV, we report a measurement of this quantity from di-leptonic $$t\\overline{t}$$ t t ¯ events where the top quarks decay into a W boson and a bottom quark. We can distinguish muons originating from W bosons and those originating from an intermediate τ lepton through the muon transverse impact parameter and differences in the muon transverse momentum spectra. The measured value of R ( τ / μ ) is 0.992 ± 0.013 [± 0.007(stat) ± 0.011(syst)] and is in agreement with the hypothesis of universal lepton couplings as postulated in the standard model. This is the only such measurement from the Large Hadron Collider, so far, and obtains twice the precision of previous measurements.
Publisher: Springer Science and Business Media LLC
Date: 07-2021
Abstract: A search for long-lived particles, which have come to rest within the ATLAS detector, is presented. The subsequent decays of these particles can produce high-momentum jets, resulting in large out-of-time energy deposits in the ATLAS calorimeters. These de- cays are detected using data collected during periods in the LHC bunch structure when collisions are absent. The analysed dataset is composed of events from proton-proton collisions produced by the Large Hadron Collider at a centre-of-mass energy of $$ \\sqrt{s} $$ s = 13 TeV and recorded by the ATLAS experiment during 2017 and 2018. The dataset used for this search corresponds to a total live time of 579 hours. The results of this search are used to derive lower limits on the mass of gluino R -hadrons, assuming a branching fraction $$ \\mathcal{B}\\left(\\overset{\\sim }{g}\\to q\\overline{q}{\\chi}_1^0\\right) $$ B g ~ → q q ¯ χ 1 0 = 100%, with masses of up to 1 . 4 TeV excluded for gluino lifetimes of 10 − 5 to 10 3 s.
Publisher: Springer Science and Business Media LLC
Date: 07-2021
Abstract: A search for supersymmetry in events with four or more charged leptons (electrons, muons and τ -leptons) is presented. The analysis uses a data s le corresponding to 139 fb − 1 of proton-proton collisions delivered by the Large Hadron Collider at $$ \\sqrt{s} $$ s = 13 TeV and recorded by the ATLAS detector. Four-lepton signal regions with up to two hadronically decaying τ -leptons are designed to target several supersymmetric models, while a general five-lepton signal region targets any new physics phenomena leading to a final state with five charged leptons. Data yields are consistent with Standard Model expectations and results are used to set upper limits on contributions from processes beyond the Standard Model. Exclusion limits are set at the 95% confidence level in simplified models of general gauge-mediated supersymmetry, excluding higgsino masses up to 540 GeV. In R -parity-violating simplified models with decays of the lightest supersymmetric particle to charged leptons, lower limits of 1 . 6 TeV, 1 . 2 TeV, and 2 . 5 TeV are placed on wino, slepton and gluino masses, respectively.
Publisher: Springer Science and Business Media LLC
Date: 12-06-2023
DOI: 10.1140/EPJC/S10052-023-11573-0
Abstract: This paper presents the observation of four-top-quark ( $$t\\bar{t}t\\bar{t}$$ t t ¯ t t ¯ ) production in proton-proton collisions at the LHC. The analysis is performed using an integrated luminosity of 140 $$\\hbox {fb}^{-1}$$ fb - 1 at a centre-of-mass energy of 13 TeV collected using the ATLAS detector. Events containing two leptons with the same electric charge or at least three leptons (electrons or muons) are selected. Event kinematics are used to separate signal from background through a multivariate discriminant, and dedicated control regions are used to constrain the dominant backgrounds. The observed (expected) significance of the measured $$t\\bar{t}t\\bar{t}$$ t t ¯ t t ¯ signal with respect to the standard model (SM) background-only hypothesis is 6.1 (4.3) standard deviations. The $$t\\bar{t}t\\bar{t}$$ t t ¯ t t ¯ production cross section is measured to be $$22.5^{+6.6}_{-5.5}$$ 22 . 5 - 5.5 + 6.6 fb, consistent with the SM prediction of $$12.0 \\pm 2.4$$ 12.0 ± 2.4 fb within 1.8 standard deviations. Data are also used to set limits on the three-top-quark production cross section, being an irreducible background not measured previously, and to constrain the top-Higgs Yukawa coupling and effective field theory operator coefficients that affect $$t\\bar{t}t\\bar{t}$$ t t ¯ t t ¯ production.
Publisher: Springer Science and Business Media LLC
Date: 18-07-2023
DOI: 10.1140/EPJC/S10052-023-11437-7
Abstract: A search for a WZ resonance, in the fully leptonic final state (electrons or muons), is performed using 139 fb $$^{-1}$$ - 1 of data collected at a centre-of-mass energy of 13 TeV by the ATLAS detector at the Large Hadron Collider. The results are interpreted in terms of a singly charged Higgs boson of the Georgi–Machacek model, produced by WZ fusion, and of a Heavy Vector Triplet, with the resonance produced by WZ fusion or the Drell–Yan process. No significant excess over the Standard Model prediction is observed and limits are set on the production cross-section times branching ratio as a function of the resonance mass for these processes.
Publisher: American Physical Society (APS)
Date: 23-10-2023
Publisher: Elsevier BV
Date: 08-2023
Publisher: Springer Science and Business Media LLC
Date: 11-2017
Publisher: American Physical Society (APS)
Date: 13-10-2023
Publisher: Springer Science and Business Media LLC
Date: 11-2020
DOI: 10.1140/EPJC/S10052-020-08469-8
Abstract: This paper presents a search for direct top squark pair production in events with missing transverse momentum plus either a pair of jets consistent with Standard Model Higgs boson decay into b -quarks or a same-flavour opposite-sign dilepton pair with an invariant mass consistent with a Z boson. The analysis is performed using the proton–proton collision data at $$\\sqrt{s}=13$$ s = 13 TeV collected with the ATLAS detector during the LHC Run-2, corresponding to an integrated luminosity of 139 fb $$^{-1}$$ - 1 . No excess is observed in the data above the Standard Model predictions. The results are interpreted in simplified models featuring direct production of pairs of either the lighter top squark ( $$\\tilde{t}_1$$ t ~ 1 ) or the heavier top squark ( $$\\tilde{t}_2$$ t ~ 2 ), excluding at 95% confidence level $$\\tilde{t}_1$$ t ~ 1 and $$\\tilde{t}_2$$ t ~ 2 masses up to about 1220 and 875 GeV, respectively.
Publisher: American Physical Society (APS)
Date: 11-10-2023
Publisher: Elsevier BV
Date: 11-2023
Location: China
Location: France
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Ludovica Aperio Bella.